my most common theme on this site is coming up with stuff that works, or sometimes doesnt... a Schrader valve in the mast and then a pressure/soap test sounds like it would work.. cool
I don't think that it would matter too much where you drilled the port because people are always attaching accessories to masts anywhere they work best- top, front, sides etc.
On the other hand, a pair of hip boots and neoprene fishing gloves would take the sting out of a Lake Michigan water test if you don't have access to a little bit warmer pool or pond.
Good luck!