BIG PHEW and a sigh of ***********. Guess how I solved this oh so pressing issue... I read the fu°Cking seatpost. Yup, under some grime and heavy scratching from removal, it turns out there's some engraving of the numbers. 27.2 baby, alright! Guess my eyes aren't so bad.
(or maybe it was just the feel that wasn't so terrible that I couldn't insert it.) I spent time trying to measure it and I was like .25 off either way depending on minor this that, so I can say a clothing measure is not sufficient for any reasonable accuracy, anyway.
Thanks for the advice, and sorry for wasting your time, but maybe some poor sap will read this and learn a thing or two about reading. I wonder if it's possible that it's still a 27.0 and the previous owner stuck the wrong post in there... It still was strangely hard. Regardless, I won't worry as much.