I am confused by what you wrote: Do you want this to be only possible for communities you maintain? If so, on DW or LJ?

I think crossposing to a community you maintain on DW / a personal journal on LJ might be possible (and I would vote for that), but Dreamwidth has no way of determining whether you are an admin for a community on LJ, as far as I know (I think at this point we can't even determine whether an account is a community)?

Sorry if this comes across confused...