You are correct that it's readtables you want, but they are an mzscheme feature; unfortunately, there is no way (yet?) to use them in Arc. Using readtables, you can customize certain things, but I don't know that there's an easy way to do $(...) because of how they work (though I am by no means an expert). If you want to hack on ac.scm or brackets.scm to extend the readtables, the best resources I can recommend are the PLT Scheme language manual (the relevant chapter is here: http://download.plt-scheme.org/doc/mzscheme/mzscheme-Z-H-11....) and just Googling for it. (I didn't have very much luck when I tried, but go for it all the same!)
Also, note that intra-symbol syntax (a:b, a.b, a!b, ~a) is implemented separately in ac.scm, in the functions expand-ssyntax and expand-compose (though you also have to modify some predicates if you add things, e.g.ssyntax?). Again, this is not (yet?) available in Arc.
I do hope that ways to extend syntax will be available in Arc soon, but they aren't yet, and I couldn't figure out readtables well enough to generate a workable solution. Hopefully arc2.tar will include this. Good luck!
