Well, as a long-time Schemer (and very occasional ChucK recreator), I
find ChucK to be a very unfriendly programming environment.

Really? Well, it's supposed to be friendly and welcoming so something is wrong there. My tips would be;
*Download and read Ge's doctoral thesis. More so than the manual it explains the underlying concepts and reasoning, the "why of ChucK", if you wish. IMHO this is a undervalued document for understanding ChucK; http://ccrma.stanford.edu/~ge/thesis.html

*List what you feel is wrong on the list for debate and potential fixes or perhaps explanations on why these things might make sense after all; I don't know and I can't help if you don't explain what issues you have.

*Have a look at Supercollider, some people who feel ChucK is unfriendly feel right at home there (and the other way around, of course). I wish I knew how that worked. I suspect it has to do with how people reason and so far nothing seems to help as much as simply helping people pick the platorm that will make them happy.

Of course it should also be said that ChucK uses some odd/new ideas that may only properly sink in after having used them for a while; I'm not sure how much ChucK you already wrote.

I'm
curious if there's source code available. I'd like to hack on it. My
apologies if I've just been blind while browsing the various bits of
the ChucK web sites.

Yes, absolutely. You can simply grab the source, read it and edit it to your heart's content. Under some conditions you can release the result. The license comes with it. Click here; http://chuck.cs.princeton.edu/release/ ,note the source is the same for all platforms, and get that. You will need a C++ compiler, some related tools and a few libraries. Then again, if you dislike ChucK and come from a Scheme background C++ may not be your idea of fun. I'm not sure anybody thinks C++ is great fun.

Best of luck whatever road you take; I'd be happy to help where I can but that will mostly be ChucK stuff as that's my main expertise, programming-wise.

Yours,
Kas.