IthaDan 

The holes on the insides of the stays are to vent welding gasses, otherwise the frame would be a pipe bomb.

The bracket above the middle of the brake is indeed a hanger for canti brakes.

With no mount holes on the dropouts, you'd be stuck with p-clamps around the seatstays, which isn't impossible and certainly sturdy, while up front you could attach to the hole in the seatstay bridge and clamp around the (presumed) chainstay bridge.

Another option is a fender that clamps to the seatpost and over the tire (google "backscratcher fenders"). These aren't as good in a total deluge as a full fenders and you'll certainly still get wet shoes, but for keeping the skunk stripe off the center of your shirt, they're than adequate. Plus, they don't have the mud clogging issues of full fenders.

FWIW, It might be a tight fit with knobbies, v-brakes and fenders all trying to occupy the same space. Full fenders would ride between the cable of the brakes and the tire.

Also, there are some fenders which really just keep crud out of your eyes which attach to the downtube, again keeping mud issues at bay and letting the fork telescope.

Edit: I feel like I should bulletpoint this, it's all fragments.