how i remember it (and disclaimer, this might be dated so take it with a grain of salt) is that in the gunz files, the interface is essentially a bunch of images packed into a compiled file (.gz or .fg or whatever it is). im not sure if anything similar to what you've suggested has been added recently, but this would just bloat the game files if it were added as a toggleable option.
what they could do is release the ijji interface skin and you can just copy and replace the one in the game files. but then again, this might not function correctly if the button placement differs from the ijji interface. anyway **** do i know. im no dev
edit
nevermind, i just actually read your post and you mentioned they've got a toggleable option to change hp bar. i guess this would work then, but depends if they want to increase the game size
