...But the certificate doesn't clearly define the computational rules for what the hotel is using. Besides, there are usually multiple rates in play - including one on Marriott.com that usually isn't quite so bad.

A lot of hotels will give you a rate that you'd get if you actually did a real currency conversion. That's the "bad" rate we frequently see. I use enough of these certs that I've just given up on arguing the exchange rate or how they post the certificate. (The back of the cert clearly states that is should be posted as a credit, but at least half the time they get posted as an adjustment. Since in most cities the hotel taxes are in the 15-20% range - roughly the same percentage I would have earned in MR points on the revenue - I consider it a wash and don't fuss about that either.)