I'm interested in a 100% better alternative to dental dams. The obvious solution is rubber shorts (perhaps washable?) does this exist somewhere?

No. There are devices like a short dam but will not protect same as standard rubberdam.
Rubber Dam. The rubber dam is excellent for isolating teeth during dental procedures . It keeps the teeth dry during the restoration phase. It also keeps water and particles from entering the throat. They are made of latex and latex free materials. Alternatives are suction devices that are placed around the teeth.
No Shorts. If you don't like to have the rubber dam placed, your dentist could use a different system(i.e. Isolite) to reduce the contamination during dental procedures. The dental dam is the only one that will completely protect you as well from falling parts such as files during root canals and is required for such procedures. Can't wash plastic, it must be sterile when it goes in your mouth.