Is it possible to infuse over car body filler

I have made a mold I used alloy as the plug and the gel coat has come away from the alloy

Its to big to put gelcoat in so im thinking filler

The part wont be seen as most of it is under the car

Obviously don't want to make the mold again

So is this possible ?????

In a word no, the epoxy will adhere to the filler.

what if I paint it ???

OR really really wax that area up ????

As above Epoxy and Poly for that matter will stick to the filler like pooo to a blanket. If the surface finish isn't that important then you can cover the face of the mould with bagging film using spray adhesive. ( after filling with bog) This will give you a surface that will release but the surface finish of the part won't be pretty. Brown packing tape and cling film also work but again the surface finish of the part will suffer.

Carbon Copies Ltd

I may try filling with bog then some more gel coat,

If you just apply gelcoat it won't cure.

Carbon Copies Ltd

You can repair the part with filler then paint with 2 pack primer and top coat, make sure the 2k paint has fully cured before using the mould if not you will have the same problem.I know this from experience 

I have decided to remold it 

on one of my moulds I got some wrinkles in a very visual area (front bonnet) and I repaired these with filler, I then easy leased like mad and the part came out perfect.

I had a similar problem (twice) and both times I filled the void in the mould with filleting wax.  On pulling the part some of the wax comes out on the part, some stays in the mould but for one or a couple of pulls its not that much work compared to remaking the mould.

HTH
Stuart