Been thinking about a simple electric reverse. It's prolly too simple not to be discussed before but...
I know a normally installed starter in a car can easly tow the weight of the car, like when you have a bit too much to drink and you try to start and don't dip the clutch when you're still in gear, you know what I mean...

Is this also true on a BEC and if so, to get an electric reverse, isn't it just a matter of switching the polarity of the starter engine and use it for reverse as well as for starting?

i believe that its normally connected to the diff, so that the engine doesnt have to turn as well. but essentially, yes.

If it's a series wound motor (most starters are) reversing the polarity won't reverse the rotation. you must reverse either the field or the armature, but not both.

Starters also have an over-run clutch that freewheels when run backward.

And it was already said electric reverse usually works better on the diff.