Ultimately, legality/GPL doesn't really enter into it - are you really going to sue them if they release it? Do they expect that, or would they just redistribute it regardless?
Frankly, if someone wants to work with you to make something awesome then they're very likely to respect your wishes and not distribute to the world before its ready. Sure, they might distribute it if you disappear off the face of the planet, but that should be considered a good thing in the potential event of your death.
Ultimately, you just have to trust them to respect your wishes. The only difference the GPL really makes is that they probably have your sourcecode too. Sure, they'd have a legal right to redistribute, but anyone who actually uses that right in this situation is basically a dick.
also:
https://www.gnu.org/licenses/gpl-faq.en.html#InternalDistribution
(I'm not entirely sure where it actually says that in the gplv2, but hey...)
Alternatively, you can distribute your non-derivative changes as a gpl-incompatible library. They can use+compile it themselves but as they can't relicense your code, they can't distribute the result. Obviously there's nothing physically stopping them from doing so anyway, or just violating your gpl-incompatible license too. I'm guessing you'd have a legal basis to sue them (I'm not a lawyer, so that's JUST a guess), but it still won't get you anywhere.
Frankly you're just going to have to trust them. Whether its gpl or proprietary doesn't really make any practical difference unless either you're rich+willing to sue(ie: you're a dick [for lack of a better word]), or they don't respect your wishes (they're dicks).
I know its much easier said than done, but just don't work with dicks. :)
The other option you have is to go with cross-engine standards like eg iqm. They can make the model work fine in some other engine, and you can then test that everything else works in your engine.
Standards are great for not bothering to create your own content. :)