The problem with this, and ANY subforum idea is that overall each subforum would pull far less traffic than a standard P&M blogs section.
A second issue you face is the smaller games that have fewer players (this goes back to the less traffic idea). There is a VERY small Malifaux community on Dakka, and the malifaux subforum gets very little traffic as it is, so a painting section of that would get even less (as not everyone posts what they paint on dakka).
Third, not everyone just does
40k blogs. Yes the majority of them will just be a single game, but if you look at mine:
http://www.dakkadakka.com/dakkaforum/posts/list/436407.page
It's very clearly a Khador blog in name and first post, but I quickly switch to Skorne and Brushfire (a completely separate game). Yes I could make separate threads, but at the same time my blog gets little traffic (I haven't posted pics in awhile). and i I were to stop painting my Skorne, that thread would get 0 traffic as it would fall off the face of the first page due to other projects/threads.
And finally, the search function is always improving, so there should never really be concern about trying to find what you're looking for.