This inexpensive homemade valve can be used to relief compressed air from a hull. I guess it is easy to modify for your specific purpose.

As you are brainstorming, and I have no idea if this would work, but how about covering a hole with a waterproof breathable material like Goretex?

Gareth

This is what is done on the floats (amas) of Corsairs/F-boats. My F-27 has one on each float near the front beam.

In case anyone is wondering which direction I've headed:

After looking at all the ideas we've generated, I've decided to go with my original plan of drilling a hole and using duct tape if I want it covered.

Since duct tape was invented for waterproofness reference, the history of *DUCK* tape that's good enough for me. I don't really expect that I'll need to cover the hole, but may do it on long outings, only after the hulls have have cooled. (my water temperature average is somewhere in the neighborhood of 60ºF)

I've drilled and refilled a 1/8" hole and will re-drill with 1/32". Not sure if I'll get enough flow through there; tests will tell. If not, I'll go with something larger. If I have to go larger than 3/8", then I'll probably just mount a standard drain hole, high on the transom. (probably the windsurfer type that was mentioned early in the thread)

Test reports will follow, in time.

GARY