Having received a lot of training in the dark arts of Scheme, this idea is the most tantalizing to me so far (can't believe no one picked up on it yet!), but:
* I'm not sure how it'd fit in with Lua philosophy. In its favor, it's an extremely powerful generic tool upon which a lot of things could be implemented; this seems to me in keeping with the rationale behind Lua's generic structures such as tables which support a slew of different programming paradigms. However, would we just be muddling Lua's identity up too much with the realm of functional languages at this point?
* I'm not sure offhand how costly the implementation would be (I'm guessing it'd be pretty non-trivial)
* I'd like to brainstorm up some potential use cases to convince me it'd be worthwhile =)
As an appetizer, I've seen Scheme turned into a logic programming language (along the lines of Prolog) by a few crafty professors/instructors (Dan Friedman, Will Byrd, Oleg Kiselyov) with a clever but comparatively small set of Scheme macros.