I did actually find a solution for this, albeit a poor one. My apologies for not posting earlier.
Supposing you have a small stack of Wallstreet parts machines unlikely to ever have hinges again, their display bumpers look identical and are just about the right size. Their length needs to be cut approximately in half and there's a slight difference in diameter so some very light adhesive helps to keep everything together. The end result looks as close to original as I'd expect to be reasonably achievable. I haven't seen this rubber deteriorate on any of my G3s yet, so hopefully it'll last a while.
In the longer term, it is possible to 3D print rubber-like plastics. That seems like a promising way forward.