Yep. The Unimould tooling resin is polyester based anyway, so there's no problem there. I would recommend still using a vinyl ester skin cost for the first layer of fibreglass, as this will give a better substrate for the polyester resin to bond to than the vinyl ester gelcoat.
But if you only need one or two parts from the mould, you should be ok doing the complete layup with polyester.