Psean wrote
The resistor across the switch would be going from the wires used to cut the coil, to ground (or am I mixing things up?), leaving some of the cut coil in the circuit right?
Exactly that, yes. Use one per pickup so you can tweak the values for each pickup to find the right values to get your preferred amount of power and low end. Added bonus is it adds back some of the
The best option is to use miniature preset pots instead of fixed resistors - then you can experiment and dial in the perfect values.
Would that make it useable on vintage output humbuckers?
Yup. Just one coil alone of a vintage 7-8K humbucker will be 3.5 - 4K (assuming more-or-less balanced coils), making for a very bright, underpowered single-coil. By adding some of the second coil back you add some of the power back and tame some highs.
In contrast, a modern high powered 14K + 'bucker will cut to about 7K - which is already in the right range.
And yes, you can extrapolate it a step further and use a pot to dial in anywhere from a single-coil to full humbucker on the fly, while playing. The spin-a-split mod as it's sometimes called. In practice I've found this to be a bit fiddly though.
[EDIT: Choklit beat me to it!?]