Don't know if the regulations didn't allow 4wd but 4wd does add a lot of weight which would reduce the advantages.
There are 4wd entries in the history of Le Mans (Porsche 959-based 961 for example) but I can't think of a 4wd prototype.
LMP has a minimum weight though, so a 4WD prototype could potentially weight the same as a MR one. The moment of inertia wouldn't be as good though most likely.