Hi Tom! I completely understand your doubts and if I were you I would see the following options, if you ABSOLUTELY need this PaymentGateway and you can't work without him, you can:
1) if you have time to wait until more users request this Payment Gateway and Jigoshop Team dedicate themselves to make it - if you have short deadlines for your project this option is (of course) not good for you.
2) if you want this PaymentGateway to be finished ASAP and you want by any means to use Jigoshop plugin (as this is the most promising WordPress web shop plugins with constant improvements and great support :-)) and you are wiling to pay certain amount of money to freelancer to programme it for you, than I can give you a few names to ask them for offer. Who knows, maybe the guys would be interested, the price wouldn't be high and you can pay it and have your PaymentGateway? You won't know if you don't try it.
3) If none above is OK for you, then, yes, you are blocked and your options are narrowed. In that case your client is already "chosen" web shop plugin he wants, unless you can give him option of usage many other PaymentsGateways that exist in Jigoshop (around 30!)
In each case, I wish you a good and successful project!