It won't do this for content edits, though, just things that touch the access level.

I'm not sure what you mean here: I have my LJ default to flocked, and if I post a public post on DW, I will get

DW-public
LJ-friends

If I then edit the public DW post solely to fix a typo, I will get

DW-public
LJ-public

And have to manually edit the LJ post back to friends. This is how it is intended to work, and really the only way it can work coding-wise, I think, because of how the differential access works.

Editing just tags on the DW post will not affect the LJ post security, or the LJ post's tags.