I've toyed with this idea before, though I had considered Lua due to its specialization in embedded environments and relatively simple sandboxing.

Interesting choice, here, in going with Ruby. Ruby is not exactly intended for embedding, and I imagine that a veritable mountain of holes in the script sandbox would need to be plugged before anything like this could see mainstream adoption. On the other hand, people are likely to be a lot more excited about native Ruby support in a browser than they would be about Lua.

Overall, I wish this project much success. It will be a long road, but I hope Ruby is just the first language to be supported in this way...