I WAS WRONG ABOUT THIS BEING PATCHED BECAUSE I'M A MORON AND I ALSO STOPPED PAYING ATTENTION TO THIS GAME AFTER 1.05
Made this as an example for someone, I'll probably just leave it up for whoever wants it.
The inputs:
Cast the buff (facing away from target)
Lock-on to target
Open weapon inventory
Swap to desired weapon
This example is specifically for "tumble"-buffing w/ full spell buff. The swap itself has other applications, but you should ask someone else who is more knowledgeable on the topic than I am.