Now you have two problems. Instead of a Lisp runtime in C, the runtime is moved experimentally to LLVM. This solves not the problem to modernize the Lisp runtime (concurrency, better compiler, ...).
It's certainly more complex, but I think could offer substantially more bang for the volunteer 'buck', as it were.
Much of the problem is already working and actively being developed and hardened against a variety of applications. The runtime system and JIT in particular. Also some existing work in functional languages using LLVM, including lisp.
But RMS has already shot me down on the licensing issue. I'm suggesting dotGNU next. If you can't directly leverage a big project, why not feed off its good ideas in a clone? :-)
