Peter, I ran the exhaust out at the scale location on a Royal Bearcat I have. It worked fine as far as engine performance but if I had it to do again (and I just might) I'd just run it out the bottom of the fuse and save the scale exhaust for a gasoline powered plane.
The glow fuel makes a real mess and it even gets in under the wing saddle. Decals are starting to flake even though they're fuel proofed. The side of my plane is just drenched with oil after every run.
Just something to think on...