yeah, it’ll look really ugly, but do the happ bat tops screw off? Or can i just remove the shaft and put in the shaft from a sanwa, or preferably the one from my hori ex2 that i disassembled?
http://www.happ.com/joysticks/competition_joy_e.htm
I’m fairly certain the competition stick/shaft is all one piece. However nothing is impossible - like you said. The only thing holding you back will be your creativity. However its going to require a fair bit of modification regardless.
On account of the whole P360 fiasco, you may be able to find a Wico shaft with built-on balltop that fits into a Competition joystick.
Coming from someone who appreciates both American and Japanese sticks, I cannot imagine enjoying a hard-spring lollipop or a soft-spring bat. Thinking about the former makes my hands sore, but the latter makes me feel frustrated and nauseous. Each to their own, I guess! 
wico shaft =9,5mm
iL=10mm
shaft it will crack on pivot
it was it been possible to alter battop from wico on balltop. you needed: file, drill and screw - stock it is easy, foto later
edit
Maaan kowal, you know your stuff. 