I weigh 270 lbs. Is the mish out the question for the providers? Most offer others so I do that, but am curious how bad it is for the woman.

Well,
If you're curious about how bad it is for the woman you may just have the answer in the question.
Do you do mish using your arms as leverage?  that may help, but if you don't imagine how it would feel being pinned , not able to move, breathe, ( weight pressing down on diaphragm and ribcage),and having almost 300lbs on top of you.
Talk to the provider about your weight and how to manuever so she is comfortable in mish.

It can be done, you just have to be a little bit creative. I love the terribly underrated missionary position because I like locking eyes with a guy when he O's. Its just sexy to me, so I don't care how much you weight, mish is on the menu, and with a bit of creativity it can be just as much fun as any other position, if not more so :o )