I've revamped the interface for Boost.Extension, to make it more concise. The main difference is that the RTTI requirement is easier to avoid now.
You can have separate containers for each set of constructors for a specific interface.
You can declare your own type of type info - perhaps just integers or strings.
The latest change makes option one easier.
A recent checkin in Boost.Build seems to have broken the build for documentation. If you have trouble generating the documentation, you may have to wait a few days until I check the latest docs into svn.boost.org in the sandbox. I managed to get the docs compiling successfully on Ubuntu, and should have the updates posted in three to four days.