I don't remember seeing any sportives banning MTBs and I don't think any British Cycling registered one would but some will discourage it. There might be some sportives that do ban MTBs because there seem to be a few strange ones out there. Check the terms before you enter.
If any medals offered are important to you, check you can complete the course in the time on that bike. Otherwise, it doesn't usually matter much if you roll in after the official finish has packed up - slow riders get swept off the course at RideLondon but I'm not sure if even other closed-road events do that. So give it a try if you like the idea.