Basically, Red, given the parameters you’ve set, you can already do that 🙂
Just use the Photoshop files, import the male layer in the female base (or the other way round), do what’s necessary to make it fit (yeah, that’s the tricky part ;P) and voilà !
As I recall, I tried to do that to build a caped female super from the male base and even though I didn’t pursue the effort, the raw result was at least usable. It should be somewhere in the forum’s archive (at the time, I wasn’t even working from the PSD files so well… ;P ).
OTOH it sounds difficult to have items usable by several base since they are different size and are using different perspectives. That would only work if the same artist did everything at once, and if the involved bases could accept such items (if they don’t that would mean the artist would have to devise even more items).
Imagine a “Shoes” pack. I may be wrong but I’m pretty sure that would mean one pack for every season 1 base since they are all different, plus one for every season 2 base, plus one for every season 3 base. That is if they are not to be restricted by sex, race, etc… That would be a lot of work (and shoes) for one pack.
Maybe using the drawing board is still a lighter more actionable way out ?
Though I’m a guy, I live with one and a half pair of shoes so maybe I used the wrong example ^_^