I personally don't like the idea of "version-per-subproject". Managing these versions without Maven can be very ugly. There are still a lot of developers who download Tapestry binaries manually and put them into their version control system.
Another idea of increasing the developer activity would be a so called "sandbox" or "incubator" project (see MyFaces or Eclipse). Some cool stuff can be developed in the sandbox. Reaching maturity this stuff can be moved into main production.
