I shoot Instax Wide (and 600/SX70 Polaroid/Impossible) every so often, but I use a Speed Graphic and put the individual pieces of film into a 4x5 film holder. Once shot, I put them back into a pack, then into an Instax camera and cover the lens so it doesn't get another exposure.

Processing through another format wouldn't work, Instax is the widest of all of them and I believe the Polaroid spec for space between rollers is not the same as Instax.

The rolling pin/pencil for processing does sort of work, but it's not reliable enough for me to want to use it.