I do know as a general rule a lot of trainers won't allow the Serpas. Most I've talked to will allow them if they know you (and know you won't shoot yourself) or if you can prove that you won't have an ND. It is a "design flaw", it's human nature for your finger to want to go into the trigger guard, especially during fast/high stress draws.
Now, why that was posted here in the first place is beyond me...I don't think he asked about how safe it was.
I don't know if it's been done, I wouldn't do it. I don't like modifying plastic holsters for different guns. Too hard to tell if something might get caught in the trigger guard or if the retention has been effected. Easy with leather since you just buy a holster a little small and let it loosen up to your gun.