All those spring pins do is ensure the axle won't slide off the spline, so you could shove pretty much anything in there an it'd work.
So, I'm thinking an appropriately sized bolt and a couple of nuts backed into each other would work,or even a bolt with a lil' hole drilled in for a R/C car body clip or the like. I'd choose a size that would ensure there's no slop, or even go a little small and put some rubber tubing around the bolt so it fit snug but comes out easy.
The drilled bolt and clip would make pulling/adding an axle take like 10 seconds. Even with some nuts on there it'd be fast, I'd rather break out a couple of 12mm wrenches then lay in the dirt and beat on a roll pin.
I'd keep the pins in the axle that stays put of course. The only hangup I can see is making sure the bolt (or whatever I use) clears the trailing arm as the axle spins.
Anywho, I'm SURE I'm not the first to think of this, so somebody come and tell me it'll work. Or tell me it won't so I can try it anyway.












