Would DotGNU work? Ignore C# and put in a elisp frontend. The benefit being that we can watch what the larger efforts are doing in those fronts, and pick-and-choose the good ideas with little effort on our side.
DotGNU would get a boost of support from emacs, and it's a familiar enough environment that a lot of programmers would be willing to go in and hack on it. They'd often already have transferrable experience on the proprietary versions of DotGNU, but would be able to vent their innate need to hack on dotgnu instead of the more proprietary versions. Emacs has a huge user footprint, and leveraging it to boost another project would help. Especially a project that rides rather close to a very popular proprietary platform - that's a lot of hackers to harness.