VOG  runs another channel that does a lot of (artistic) metal casting with mostly 3D printed positives, called "lost PLA casting".
He uses casting plaster and built his own vacuum chamber and furnace to improve the quality of his casts.
First off, Pilchuck is amazing, and if you're at all interested in glass, you should seriously consider taking a course there.
Second, modulo a few tricks for clean burnouts, if you can print it, you can investment-cast it in whatever material you want. It's amazing how powerful cheap printers become when you combine them with a little casting skill - you can access nearly arbitrary geometry in some useful, high-performance materials without a $100k printer.
(I have an Ender 3 printer, and a microwave, so it looks like I can possibly get started for <$100. But both because of price and turnaround time, I am not very interested in using online resin printing services.)
Unfortunately, most kiln-casters prefer very high lead glass (40%+), because it flows very well and doesn't devitrify easily. This, however, makes coldworking an environmentally-dicy proposition, as it generates lots of micron-scale lead dust.
A microwave may suffice to fuse small glass jewelry (...kinda, the annealing will be pretty horrid...), but isn't going to cut it for casting. Stuff at the scale presented in the article would require firing times of at least 12 hours, with much better temperature control than a microwave can provide.
Another approach is vapor smoothing - you can use acetone vapor to smooth ABS prints. Acetone won't do anything to PLA though.
I've heard of other chemicals being available for smoothing PLA, but they're nastier than acetone and I don't know anyone who's actually tried them.
ABS is a very bad material for casting if you're planning on burning it out, but vapor smoothed parts do make very nice plaster molds if you can remove them. Chloroform works very well for PLA, but it is much much nastier.
Either way, feel free to send me an email ping if you like, I'm always looking for folks to share knowledge with.
Is there a mathematical term for solids with this property? It reminds me of the vertical line test, but in 3D.
But yea, certainly you could add a leather wrap underneath.