Whitey, if you don't mind making a reversible modification to your 122 spool, you could split a 120 spool, hollow the center to fit the 122 core, and hot glue it in place on the 122 core (carefully aligning the slots so you can still load the film). This would still need to be unloaded in the darkroom (because of the cuts in the 120 flange) but would keep everything well aligned.
If an irreversible modification will work for you, you can cut the 122 spool, drill holes in the ends of the 120 to fit them, and glue the 122 ends into the 120 to make the combination the correct length and the 120 centered. I've recently done this with a 116 and it worked pretty well. As a bonus, you don't have to unload in the darkroom (though you can only shoot one roll on an outing, unless you modify more than one 120 spool).
You could also cut the 122 spool, make keys to fit the 120 drive slot, and glue or solder them into the ends of the 122 core, and have an adapter that will allow you to reload in the field just like you would with any other 120 camera. This is the most versatile, and the one I'd choose for my own use if I'd thought of it before making the above modification (I might still do it -- I used hot glue, so I can disassemble the unit I made with a little care).
BTW, with 6x12, if you drill a 3/8" hole in the camera back and pressure plate, centered from side to side and 3 cm from the takeup end of your frame mask, you'll be able to use only the odd numbers on the 6x6 framing track and get 6 exposures per roll. Drill 3 cm from the supply end instead, if you'd rather, and then use only the even numbers. You shouldn't even need the red filter, but you can glue some Rubylith or similar material over the inside of the hole if you prefer.