Yes, I used Semagic for a while, but quickly got frustrated with how many DW-specific features did not work (biggies, being able to manage one's friendslist in the client and crossposting in the magic way that DW does it) and eventually gave up and started using the web form. I don't recall there being a way to mass-edit post security with it. IIRC LJ-Sec got confused when I tried to point it at DW, though, so if I'm just misremembering and you can do that with Semagic, I'd expect similar (failed) results.