Sidi Gore-tex cycling boots. Might be overkill for what you're looking for.

I have heard good things about the Sugoi Resistor booties. I don't know how well they fit over mountain shoes though. Also, booties tend to get badly torn up with platforms unless they're designed for them.

What about Gore Tex socks? DH used to use them for mountain biking. Gore Tex hiking shoes might work too.