Yay, I can't wait to see this working! I think that it having it switched for particular actors is the best way to do that. Just to make using this feature even easier to use, I'd suggest adding "Default Lightweight Actors" (in SW preferences or Game Settings; the latter seems better because some game are more physics-oriented, some aren't) or something like that; this way if someone makes a game based entirely on lightweight actors doesn't have to remember about switching it every type when new actor type is created.
By the way, I read in the blog post that there with 200 actors lightweight option was 60 FPS fast, and I wonder how many actors it would take to bring it below this value. Not that I would need more of them, just interested. ^^'