I don't know how exactly xanthines like cafein interact with psychedelics. They're supposed to inhibit an enzyme that breaks down transmitters (c-AMP) of a natural 'echoing' system of the brain.
So theoretically, taking caffein would amplify the feedback-loops of the brain.
Hyosciamine and other tropanes do block not only signals within the brain, but also signals going to the brain.
I think that specifically thát effect is responsible for the enhancement of psychedelics, by these tropanes.
My guess would be that, by blocking signals to the brain, the brain starts 'filling the gaps', itself, and psychedelics are putting the brain intoastate where it is even more able than normally, to fill in missing information for itself.
If that hypothesis is true, then other substances that block signals going to the brain, via other routes, would also enhance the effects of psychedelics in minute amounts that are not narcotic by itself.
A small amount of salvia, DXM, or opiates would then have a simmilar effect as tropanes. Again, in amounts that do not have any narcotic effect itself.
But if those substances block more signals within the brain itself, then signals going to the brain, than it could block more psychedelic effects at the same time. So it would depend on the balance between the blocking of signals within the brain and signals going to the brain.