It was simple, really. I read somewhere in a thread - which, I can't remember at this moment - that it was possible to manually edit save files to set the character class of a player, and I believe it was Pasik himself that mentioned this. That's what got me started on my custom model binge.
In the header of .person save files, there's
soldier_group_name="default". "Default" just needs to be changed to the desired character class and, voila, custom models and loadouts
(or so I'm inclined to believe; I haven't tested loadouts yet and want to once I start working on custom weapons).
I must be blind, however, as I can't find where unlock_manager specifies the character class. Am I just not looking hard enough or is new code the solution I should be looking for?
Edit: Found the thread and I was right about the source. "Model-Specific Vest Models?":
http://www.runningwithrifles.com/phpBB3/viewtopic.php?f=7&t=2286#p13928