Yes we can do the female threads also but my only concern with that would be stripping out. We have a variety of brass fittings to be moulded so i will have a look and see what would suit
I've never seen any strip out, the threads are really coarse. Maybe 3 threads per inch? But you know better than I about your options and costs. I think the ones with the insert, as you showed, look better than an MX cap sitting above the tank, and some of those had locks I believe.
Looks like a good project, if it succeeds you can branch out into other makes and models. If I still had the crashed tank on the bike, I'd be in line to check out one of yours.
One good thing, the tank will be a lot lighter in weight. Better, lower center of gravity, and lower shipping cost.
In making more capacity, you might check clearance between the tank and frame, see if that can be squeezed? A little here, a little there, bam, a gallon and a half.