Thanks! And yes, happy to explain. The explanation is the whole trick and it's shorter than you'd expect.
A pose in GL2 is 81 numbers, the ones saveposestring writes. The mistake I made first - and the reason my mod chibified people's characters for a while - was thinking those 81 numbers are the pose. They aren't. Roughly half of them are the character: every xpos and ypos, every xscale and yscale, for hands, forearms, upper arms, thighs, shins, head and body. Plus the layer order, which is the very first number. Apply a preset pose as it is and you overwrite the player's proportions, their joint positions and their layer order with whoever the pose was originally made on. You get a different body doing the right pose.
So don't replace, merge. From the pose take the rotations, the skews and the hand shapes. Take everything else from the character being edited. That's the whole "adjusts to OC adjustments" part: the pose brings the attitude, the character keeps its body.
Two practical bits on top. I build the library for whichever character is being edited and rebuild it when you switch to another one, because the body numbers it has to wear are that character's. And I write the finished poses into pose slots the game isn't using and then tell the editor how many pages it has now, so the game's own picker draws them. That way I'm not faking a UI, I'm filling in one that already exists.
On studying the code: I'd rather not hand over the whole thing, but it would honestly be the least useful part for you. GL2MP isn't a modified GL2.swf - it's a separate program that loads the player's own copy and works on it from outside, so almost none of it would drop into a mod like yours. What transfers is what I just wrote.
Ask me anything specific though and I'll answer it properly, and if you get stuck on a piece, show me and I'll look at it. I'd rather do that than have you pulling apart a build.
One thing in return, if you don't mind: your team ships for Android. That's what people ask me for most and it's the part I know least about. Who should I be asking?