I am really quite proud of myself (I'll allow myself this indulgence as it's nearly 2 a.m.).
Matthias wrote a really nifty interface to Mac::Memory handles so that you could read and write to them as filehandles. The thing is, in the old MacPerl it relied on HandleSocket.cp, a GUSI subclass or something that would do all the dirty work. It was written to GUSI 1, and we now use GUSI 2. Well, I don't know much about GUSI, and even less about C++, but after a bunch of trial and error, I finally ported it to GUSI 2 and got it to work.
Well, it reads anyway. Writing is not working yet. Oh well, there goes my pride.