This is what I'm doing. I bought a dozen or so 'disposable' masks, but since I'm only wearing them for brief trips into shops in an area where infections are now pretty low again it seems overkill (and wasteful) only to use them once, and I doubt they'd survive a trip through the washing machine. We don't know exactly how long Covid-19 lasts on different surfaces, but AFAIK there's no reason to think it doesn't behave much like other coronaviruses and die off pretty quickly on an absorbent surface like a facemask, so I just have two pegs in my coat cupboard; one for recently used masks and one for 'safe' ones. It's not exactly best practice, but it feels sufficient atm.