Lisp is especially conducive to this sort of thing because it is (arguably) the best suited language for rapid prototyping; sure this could be done in other languages, but it wouldn't be as pretty or smooth.
I think the limiting factor is the existence of a REPL.
What you are speaking of with the AST doesn't sound very "live", but definitely sounds interesting.
Python would probably make this possible as well (if things like Reinteract are any indication).
It is one of those things were I became insanely fascinated and then suddenly found myself deluged under a ton of other fascinating things. Hopefully I'll be able to make it back to it at some point. Consolidating my art+music+coding together could open the window to doing something crazy like finishing a project...
Now, what I would like to see is how the AST of a large project evolves over time. Not just the commits to a repository, but the entire AST of the codebase as it's being typed.