...probably falls under your "tried all the obvious" courses of action, but the only way I've ever universally changed the fonts is through jrummy's "Root Tools" ...though I haven't tried it on AOKP yet.
I think I finally got it. I had taken SonySketch (desired font) regular, bold, italic, and bold-italic, and masked each of them with the proper name found in the /system/fonts file and made them into a flashable .zip but this didn't fix it. Today, I just dropped all of them into a folder on my SD card, and used Root Browser to copy-paste them over from my SD to /system/fonts and rebooted...VIOLA! The entire phone has been changed. I'm going to try making a flashable .zip again though so that it's easier to share.
EDIT: Here are the new flashable .zip's. I just took the Steel Droid 5 .zip that was on my desktop and erased everything but the fonts file then duplicated it, keeping META-INF but replacing the stock fonts in the 2nd .zip with the renamed Sony Sketch fonts. Tell me how it goes if you're interested.