Cedar as boards isn't too much of an issue. There's isn't anywhere near the surface area exposed as with shavings and if the cages are outside or in a building kept open it's unlikely to build up the volatile oils. I would never use cedar shavings or too much wood in an enclosed building such as making entire livestock stall sides out of it but framing a hutch probably won't concentrate enough of the oils to create any health risk. Pine has the same risks of cedar just in lesser amounts so it's not that cedar is unique. It's just the worst. Don't use treated or glued wood such as plywood and small boards that are made of other small pieces like some 2x2's can be where rabbits can reach it. I'm not sure I'd trust any stains or varnishes. At minimum you would need a long curing period before putting rabbits in the cage. I have used marine polyurethane. People use it to seal plywood for making coral growout tanks instead of having to use a glass or plastic material and if it gave off anything toxic all the sensitive saltwater critters would die quickly. It pretty much makes the wood equal to plastic. Waterproof, wipes clean, can be sterilized with bleach or vinegar.... However it's slightly expensive but with large cages, plenty of hay, and a few cardboard items my rabbits haven't chewed my polyurethane coated base for the indoor cage. I made that base over 5 years ago.