Wigglyamp raises an interesting point about the NY IFO not processing 337s anymore.
I found this too, when i tried to get a GPS approach approval for my plane. They gave the avionics shop (the last of the three that tried this) a right run-around, clearly not wanting to do it.
The DER route to get around this is a good one but is a lot more expensive...
Isn't there one other option which is to fill in a 337 (which needs an IA, etc) and send it off, wait for X months, and if one hears nothing, one can install it as proposed? That way, the NY IFO not co-operating would not matter.