killajb was nice enough to send me the couplers (with shocks still attached!), but sadly, they are too small in diameter to fit over the shock bodies. I'll still use them in a future project, but I was just thinking of this when the solution hit me - and it was so simple, I had to laugh at my stupidity. I'll just get a couple of those spring top bushings, and glue two together - the parts in the top right corner of this pic:
The GTP uses threaded bodies so it doesn't have those, but the ones from the Mugen (or any other similar vehicle) should work just fine as long as the diameter is right...