Any on a supplier of traditional soldering investment,
or the name and source for the equivalent product that does not
harden and can be reused, would be very much appreciated.
You might find garnet sand useful for your purpose. While it is
reusable, the downside is that it acts as a heat sink (as would
investment as well) and it's a bit messy. You mix it with water, the
thicker the better in my opinion, set up your rings and then begin
heating. The heat will dry the garnet sand, though it will never
become hard like investment. It works quite well as long as the
parts aren't tiny, like prong settings for instance; for parts like
those, investment is preferable. I purchased it from Rio Grande a
while back, though I'm sure other suppliers carry it.