This may sound silly, I am considering using a guitar head with a 2x12 bass cabinet, is this a doable setup? The head can be 16, 8 or 4 ohms.
That will work, as long as the impedance (the Ohms) of the cabinet is equal to or more than the impedance of the head. What will be different about it is that a bass cabinet is usually intended to reproduce the full frequency response that is fed to it and therefore will likely sound more fizzy and or boomy than a guitar cabinet would.
Also pay very careful attention to the impedances because I think many bass cabinets are made with lower impedances than guitar cabinets. This is because bass heads are often solid state which can more easily be designed with arbitrarily low output impedances.