Can I run a 700c/28"/29er wheel & tire on a 650B full-suspension mountain bike frame?
The only way is to try it and find out. The great thing about disc wheels is that the brakes are in the same position regardless of rim size, so if you put the wheel in and it physically fits (i.e. it goes in the frame with enough clearance) and for the rear it has the right cassette, you're good to go (possibly with some minor adjustment to the brakes). If the outside diameter of the wheel with the tire on shrinks versus your 650b wheels with the 2.25" tires, it should work fine. If it grows, you may have to worry about clearance. You can check this by putting the wheels on top of each other. 

That being said, reducing tire size may not work so well for actual mountain biking, so if you are going to take it mountain biking rather than electric biking on the road, you may want to stick with the bigger 650b tires.