Gate poly minimum L is certainly less than minimum of
(contact + 2*overlap). So it's not something that is a
high value option for digital. It would only be practical
for long channel analog stuff.
Then comes the question of what happens to the gate.
Strain effects, VT shifts from contact introducing some
metals into the electrode, reliability degradation, etc.
Somebody would have to do some work to look for,
quantify, model and worst-case the effect of doing this
abnormal thing.
Basically the foundry didn't see fit to qualify the special
case, so they say you can't do it.