I have tried manually spreading the instax developing agent with a print out of the camera ( yes, in absolute darkness ) with a brayer (more commonly called a print roller by many).
I did not have uniform pressure or speed, and the resulting development was uneven.

Which was a drag, because I was not able to get the film eject on my autofocus instax to work, and ended up scrapping it event though it was a nice instant picture camera bought very inexpensively.