Hi Bruce;
   Have you searched this out on here? I\m pretty sure this has been asked before. To me, the obvious answer is to do it just like you did with an IC model, but with a dummy radial mount to fit the motor mounts. A rear mount set up would be easy, just like mounting the motor. For a front mounted motor. I think a two piece approach with some sort of "X" that attaches to the plane's motor mounts, and then uses a center bolt to attach that to a rod like you have used before. I would need the model in question right in front of me to get more specific, but I think you get the picture. I would start with a really heavy duty replica of your motor mounting bracket and go from there. Where it would attach to to the bigger rod would need to be pretty stout and solid also.. Then just hold it in the stand or connected to whatever you did before.
  Type at you later,
   Dan McEntee

I have tried it on two ships with mixed results.  On the rear mount I just took a mounting "X" and bolted a large dowel to it in the center.  Worked OK but I was always worried about the stress w/o the beams.  I resorted propping it up nose down in a vice.  It is a "through the firewall mount which screws into the plywood doublers so I doubt it would have been weak .... but.   On the other I used the Okie printed mount and it did just fine the convectional way using the same "X" and dowel.  Pictures are of plane #1. 
Since I am in a no spray paint zone now, I will be using monokote and won't need the stand.

Ken