It would need an admin to do this as you say, but the best way is through Special:ThemeDesigner: there's a transparency slider on the far right, just above the Save button.
Looks great! I've been playing the original F-Zero, and was just thinking of doing the rotating ships thing for the sprites of the four original machines before I saw this (plus the other NPC racers... not sure if they have articles yet? and BSX ones too!). What would be the best way of ripping 2D images?