I've looked through the mod_perl source code, but am a bit overwhelmed and as far as I can tell it seems to maintain a single interpreter per Apache process.

As far as I know mod_perl 1.0 did maintain a single perl interpreter per Apache process. So you are unlikely to find anything in that source code.

However mod_perl 2.0 has a number of options for how your perl interpreters are managed, I am a little fuzzy on the details of that, but you might try checking the mod_perl site for more info, and maybe looking over the mod_perl 2.0 source code as well. You could also try asking your question on the mod_perl dev list, Stas (the guy running the mod_perl 2.0 project) is a really nice guy, and may be able to point you in a good direction.