Just to cover my bases, I asked over on the Fit Fatties Forums if anyone else had run into the MRIs Don't Fit You problem. Here's a reply:
Re. MRIs. BALONEY! There are "open MRIs" which can accommodate people up
to 550 lbs. (and some have a capacity up to 1,000 lbs.) Something like
this (http://www.washingtonopenmri.com/open-mri-maryland-obese-mri-open-m...) looks especially useful, because it would show the knee while weight-bearing. Here's more about a standard, open MRI. (http://news.msu.edu/story/1260/) I'm somewhere around 360 lbs., and I have an MRI about once every three years to monitor neuro stuff.
Okay, that's useful. This particular doctor must not've seen anything that would warrant the extra rigamarole -- I'll go with that explaination, anyway. Either way, shopping around for another practitioner, and now I know what to ask for.