i have read about such a thing and it is somewhat possible . you would have to flip the diff around inside the transmission and machine away at its casing to make clear. and you would have to find some way to lock up the center diff by either installing a locking diff or welding up an open diff for rwd, or flip the rear diff over and put in the front for awd
and yes you can make a RWD soob by locking the center diff and removing the front axles. or with older models just put in 4wd and remove the front axle shaft.
now with the rear engine thing, the width of the rear diff is the same as the transmission at the front diff, so if you managed to mount it all rear enginelike, the trans will still fit between the rear axles, and the rear diff will fit betweent he front axles
and it may not be worth "money and labor" for someone to try to build this for you.
but this is the kind of thing its worth trying to do if you have some tools, some mechanical aptitude, and willingness to learn. it is possible, its just the kind of thing you have to invent rather than buy off the shelf
if you think its easier to "go get an MR" then this kind of thing is not for you. otherwise if you study the subaru platform you can see where i am coming from