At this point im surprised the community hasnt been given the resources to make/test new vehicles and game structures/objectives in offline mode like they do with skins and maps to be greenlit later if liked. The game already has die hard community willing to put in the effort. But like you said opening that door may open up issues with hacks. Iv dabbled with skins, maps, sounds and can 3d model but i dont have a clue when it comes to implementing that model via code...
There might be people here who could make a mesh. I know we have professional level skinners.
One big problem is that I believe there is some hefty commercial software used int the hierarchical componentization of all the little parts for animation, physics simulation, damage modeling, etc. This is not open source. I assume he has a limited number of seat licenses. So maybe a dedicated intern could go learn all that and use that seat license to help build a new plane, but it probably isn't anything that can be shared generally.
Building the mesh is just the first tiny step in the process and he could always just go buy that off Turbosquid probably.
I could be wrong, but I think the issue has been addressed before by HT and it just isn't feasible that the community can just in and start adding game objects.
I had added a custom object before (observation balloons in WWI:WF) but those were statics. An aircraft model is an entirely different matter.
But if there were one or two advance experienced modelers in the community, they should contact HT and see what they could do, but I don't see a plane editor the way you have a terrain editor.
$0.02.