For folks who make heavy use of access filters, cross-posting between LJ and DW doesn't work out so great, which is actually discouraging transitioning to DW.
Right now, if you have cross-posting set up between LJ and DW, the only way to keep your filters in sync is to do so manually. In addition to being a really irritating hassle, this has proven to be highly error prone.
I have now lost count of the times that I have clicked through someone else's DW xpost link over on LJ in a locked post only to discover that I didn't have access on DW because the author had had their filter lists on DW get out of sync with LJ: I was on the filter to see the post on LJ, but not on DW. This includes cases where the author had turned off commenting on the LJ post to drive commenting to DW.
In all the cases where I have delicately pointed out, "Hey, I can see this on LJ, not DW; are you sure I'm supposed to see it at all?" the response was, "Oops! Crap! You're supposed to be on that filter!"
This is actually frustrating migrations to DW -- if when given the choice to comment on DW or LJ, and only LJ consistently works, one's friends will strongly prefer commenting on LJ, and effectively resist using DW.
This would actually be a pretty easy problem to somewhat ameliorate, if not entirely solve, for the common case of someone trying to keep a DW journal and an LJ journal in sync.
Here on DW, the interface for granting read access to another user is http://www.dreamwidth.org/manage/circle/add?user=USERNAME&action=access. This page not only allows one to grant read access, but shows the user a list of their subscription filters for optional assignment. By default they're all unchecked.
Instead, if (and only if) someone (USER@DW) has crossposting set up (to USER@LJ), and in the sole circumstance they were granting access (on tick of the "grant" box) the page could use their stored tokens (or prompt for them) to log in to LJ to see if (1) the USER@LJ is friends with an account of the same username as GRANTEE@DW, i.e. GRANTEE@LJ, and if so, (2) get the memberships of GRANTEE@LJ in USER@LJ's filters, and wherever those LJ filter names correspond to USER@DW's filter names, default those checkboxes to checked.
Put another way, if you have crossposting set up, when you go to grant access to someone, if your crosspost account is friends with someone of the same name over on LJ, DW will propose (not require, just default) to add them to any same-named filters here on DW.
Additionally: if one visits that page to //modify// filters (the target user is already granted access) it would be nice if the page listed in text one's LJ filters that user is a member of, ideally in a column next to the tickyboxes for fast visual comparison.
Additionally: if (again the original case of on granting) they are members of any filters over on LJ that don't have corresponding filters on DW, the interface should simply state them in text, along side the list of tickyboxes, i.e. "On LJ, you have a friend named 'Fred', and he's a member of the following filters."
Optional variations: Add a boolean field, "Sync filters", on a crosspost record, where if it's ticked, that crosspost relationship will try to do the above, and if not, not.
That the case of granting access to someone who has the same username on both LJ and DW, and whom one has already "friended" over on LJ, is incredibly, incredibly common. Thus anything which assists filter management in this case is actually addressing a large chunk of the problem, even if it doesn't work in other cases (see next), so long as it's not detrimental in any of them.
That the case of granting access to someone who is a "friend" on LJ, but who has chosen a different username here on DW is not all that uncommon, but dwarfed by the prevalence of the previous case, therefore it would still be helpful enough to trouble to do this. (Heck, someone could go all out and provide a field on the page in question, "This DW user is the same as this LJ user: [field]" and allow the grantor to specify, for purposes of populating those filter tickyboxes, which user the grantee should be considered the same as, making this solution extend to this case, too.)
That the case of true name-space collisions (one is friends with fred@LJ on LJ and grants to fred@DW on DW, and these are two completely different people) is both so rare and so notable, that the user can be trusted to notice that's what's going on, and untick tickyboxes as necessary.
That for people migrating between LJ and DW, the majority of filter-assignment happens when they grant to an LJ friend who has created a DW account. While people may be creating new filters on either platform and going through and adding to them at any time, that is more rare than assignment on granting. Thus helping keep filters in sync just on initial granting addresses a large chunk of the problem. While it might be nice if we could also support //ongoing// synchronization of filters, that's a much harder problem, and we can solve 75% of the problem without going to all that work.
If LJ doesn't have an API for getting at that info, I suggest scraping http://www.livejournal.com/friends/add.bml?user=USERNAME for the filter list and membership.
As I've described the feature (interface responsive to changes in field values such as the "grant access" tickybox, maybe a field for corresponding username, prompting for LJ login tokens), either AJAX is required or the functionality of the page would have to be broken across several pages.
Over the Top Elaboration Discouraged:
If we really wanted to go all out with xpost filter sync, we could have a whole interface here on DW for manually assigning correspondences between one's DW filters and one's LJ filters, as part of one's xpost settings; e.g. "my 'fam' filter on LJ == my 'family' filter on DW". That seems to me like doing 900% of the work for 10% of the benefit, so unless the previous more modest suggestion were to be implemented and prove wildly loved and important, I'd skip it.
It would be kinda neat to have a "xpost filter audit" page that one could go, that would basically diff one's filter lists, and report any of one's grantees whose filter assignments differ between DW and LJ, so one can do something about it manually if one feels like. But at this juncture that would be feature creep.
Poll #5573 DW/LJ filter sync on Grant Access for Crosspost Users
Open to: Registered Users, detailed results viewable to: All, participants: 44