I don't have a Model M, but I've hand washed a pair of Apple Extended Keyboard IIs that I bought off of eBay (even used a scrub brush), and they've worked well

I don't have a Model M, but I've hand washed a pair of Apple Extended Keyboard IIs that I bought off of eBay (even used a scrub brush), and they've worked well for years. I shook as much water as I could out of them, and let them dry outside before plugging them in.