Ok, I did a little more hacking, and made a Scheme version of the function I coded in CL earlier. Still nothing near usable, but it is enough that in about half a dozen lines of code you can hack ac.scm to use it instead of Scheme's normal read function. And because the function leaves normal lists alone, you can use it to load the normal arc.arc definitions without any problems.
(Note: if you actually want to try this, in addition to just adding (require "read-indentation.scm") and changing various instances of (read) to (read-indentation), you probably want to add (flush-output) after the (display "arc>") in the toplevel definition, or the REPL you'll get will be extremely messed up.)
