"I was told that it was possible to put a piece of thinfire paper under a circular piece of glass that is to be draped over a mould.... Has anyone done this or heard of doing it?"
Yes this can be done. Some caveats are in order though.
It is important to put a separator on the mould -normally kiln wash - before using it. Once coated, it will not need to be re-coated unless the surface is damaged, scratched, etc., for a very long time. If the kiln wash seems to be rough you can smooth it in various ways as noted in this tip. The advantage of kiln wash is that it does not cost much and lasts a long time. The thinfire or similar is a one-time-use product.
The binder in the thnifire burns away during the heat up and allows the now unbound separator to drift down to the mould before the glass begins to conform to the mould. In that way it is very similar to a fine dusting of kiln wash powder over the mould surface before firing.
I don't see the point of using Thinfire or similar during slumps (although I can see that there are occasions where this method would be useful). For drapes, placing a bit of thinfire over the mould - especially if it is metal – is an additional precaution.
My view is that you have to kiln wash the mould anyway. KIln wash is cheap and long lasting while thinfire needs to be replaced after each firing.