I don't think you'd be able to play properly using the mouse wheel for jump in RIK. Some things like doublejump and alike, rely on you HOLDING the jump key down. If you were to use the mouse wheel, then there would be no permanent "down" state, just quick "moments" where it would be down, and meanwhile it'd be in the "up" state.
- Doublejump would not be possible
- General jumping on the ground would be far less reliable in RIK compared to using the normal way of holding down a button before you land. You'd very likely end up losing speed.
- Stairs would probably have the same problem, same goes for misc walljumps and ramps.
- Possibly other negative consequences to other tricks and alike.
This is all assuming you wouldn't constantly scroll the mouse at a speed so that the game wouldn't recognize the "up" states.
Edit: as I thought, it's not blocked. It's just how the key registering and jump handling works. Which could be worked around I guess, but we're still stuck with the above reasons that aren't really quite as work-around-able.
So yeah, I fully understand the being-used-to-it argument. But sorry, I don't think there's much to be done here aside from trying to get used to space, or one of the mouse buttons possibly. At least not for the time being.