Right now, you can easily fit the fuel hydrant under a 737 if you just move the refueling point closer to the wingtip.
With other airplanes, like the MD-80, this is not really feasible, and will require a completely new kind of service with a completely new ground crew animation that, instead of using a raising platform, will use another cable a step ladder to reach the wing ( in some cases not even a step ) so, as with all the other things related to GSX features in general, this will have to wait until after we'll release GSX for MSFS 2020, which is everything we are working at it now, when GSX is concerned.