I haven't tried it, so I don't know, but in general I've found it easy to call MzScheme code from inside of Arc.
You can add xdef's to ac.scm; Anarki has a $ macro to call into MzScheme if you use Anarki; and I describe my favorite way to call into MzScheme here: http://arclanguage.org/item?id=8719
