Anything that doesn't survive the 150° C oven is not going to survive a 1500° C blow torch.
Gelatin has a melting point of about 35° C, maximum. It is a thermoreversible reaction, unlike the coagulation of eggs, which is thermoirreversible. Eggs set well in an oven, which is why they are used in so many baking recipes; gelatin does not, which is why it is almost never used in baking recipes (except as a stabilizer for fillings after the baking process is done).
You are definitely barking up the wrong tree here. If your custard is melting as opposed to setting in the oven, then there is something seriously wrong with either your technique or your recipe. The addition of gelatin is unlikely to help and, as stated above, even if it does help the custard set firmer, it then essentially becomes a panna cotta and you will not be able to caramelize sugar on top of it.
Summary: Don't pursue this. Find out where/why your custards are going wrong, and fix it.