The full album modifier is a bit of a pain, because by design, no matter where it appears in a search string, it pulls every file of any description that matches the [album] tag, over-riding any other exclusions in the string.
This means that if your album art happens to be imported and tagged, these image files would also be listed, even if you specified [media type]=audio. ~a simply ignores that and carries on regardless.
I don't think it's possible to get a list of albums sorted randomly like that without using the ~a modifier, which means you can't have exclusions.
You can get close using the ~limit modifier if you know how many albums you want, then you can add your exclusions, but will probably have to accept some form of [album] sorting to keep the albums together.
[media type]=audio ~limit=30,-1,[album],[album artist (auto)] -p="not for work" ~sort=[album]
will give you 30 albums picked at random from your library, minus the "not for work" tracks. your mileage using this approach will be dependant upon the ratio between the total number of albums in your library vs. the number of albums you want in your list results.
-marko.