Hi Rachel, I am not sure if this will help but it's worth a try.
When I made hinges for bracelets years ago I would insert a graphite
rod into the holes and have these extend on either end. The
investment would hole the graphite pieces and the metal would flow
around these "rods" during casting. You would then have to drill out
the graphite to re-establish the lumen. The graphite we would use
was mechanical pencil "lead" which at that time could be obtained in
various diameters. These were solid rods. The newer "leads" are
unfortuately not solid but instead are plasticized mixtures of
graphite which would melt during the burnout. The old forms of
"lead" may still be available, but you would have to hunt it down.
Perhaps a search on the web might uncover a source.
I thought I had, but never tried since I no longer do castings, is to
use titanium wire of the proper diameter to hold the space and after
casting try to remove it. It may be worth a try, but I'm certain that
others who do castings on a regular basis will give you
a procedure that works. Good luck, Joe Dule