John-

Not sure what year and boat model you have. Might want to add it to your signature.

We have a MKII. I assume you are talking about the fixed cabin window over the stove? If so, I can think of several reasons for not attempting this. The first is the window has a slight bow. Any openning porthole frame would be flat meaning you would have to use a thick gasket in the flange area, which might make it prone to leaking. If you torqued down on opening port flange and frame screws, it would induce stress in the cabin window - more stress may lead a greater chance of developing cracks, and the fixed window pulling free of it's adhesive seal to the cabin.

Just my 2 cents.