@Mark Obviously I don't know what kind of roadblocks you've run into development so I may be way off base here, but I do know I've seen Umberto (GSX dev) state that Leonardo did their GSX integration with the Maddog fully on their own utilizing the GSX documentation, and it fuels automatically via GSX, and then also Fragtality created the Fenix2GSX addon that did the same before Fenix implemented their own native integration. I don't know first hand but my sense from seeing Fragtality speak is that his work was either with no or little input from Umberto/GSX. I feel optimistic based on all that that the framework may already be in place for a full integration!
I also know that GSX also natively has the capability to fuel aircraft as is -- the challenge in the past with most add-on aircraft seemed to be the passenger and bags.
But I dunno, I'm just some guy, I can only report what I've seen browsing the various forums and discords.