If it were me, I'd still use something other than PTFE. Reason is, even a small amount of thermal degradation in PTFE produces some pretty toxic stuff. If everything works fine, you're probably ok, But if you have the same luck I had as a kid, sometimes things stick and the chute begins burning inside the tube and, well..
If you note that there are no baking sheets (often used for broiling) that are PTFE coated, you'll understand. Broil some tater tots on a non-stick sheet and your parakeet will die. Literally. Decomposition products (carbonyl flouride and perfluoroisobutylene and others) are lethal to budgies at very low levels. And to humans at levels a little bit higher. Seriously, the products are nerve-gas level toxic. I take dead budgies as a little warning to us non-budgies.
One other reason not to use PTFE is that drllled and tapped holes close in after drilling or tapping. Carbon filled may be better (and I think you have more experience here than I do) but getting the hole and threads just right might be tough. Although, one might regard it as a self-locking hole... Last, the strength and stiffness of PTFE compared to other plastics is like Manchego cheese... Last (really, this time) glueing the ring in may present difficulties and I could see the ring-body joint failing. Maybe I'm being too engineringish, though.
Let us know if you do this and it works. What class engine are you using?