I've taken a stab at syntax highlighting based on scheme.el: http://zardoz.net/arc.el. Rudimentary so far, but gets most of the definition and control keywords.
The "add-to-list" line for some reason is not enough to automatically activate the arc-mode for *.arc files, although I can activate it manually by doing M-x arc-mode.
By adding a variation of your snippet to my .emacs, I got arc-mode to activate automatically: