Sam, in the example I lashed up above, with the router, it would be normal to advance the router along the bed using the handwheel/rack for parallel slots, or the topslide for slots which aren't parallel to the axis of the chanter.
The headstock can be locked by various means, but for most of them, the carriage couldn't be advanced under power from the motor.
It's possible to fit various forms of dividing or indexing heads to the main spindle to both lock and position it, but the carriage advance would need to be by hand, which shouldn't really cause a problem. e.g. The Hemmingway attachment
The only way I can think to drive the carriage with the chuck (as opposed to whole spindle) locked, is to disengage both the backgear and the backgear driver crescent. This would allow the main spindle to spin, but disengaged from the chuck. The chuck can then be indexed on a jaw, and a block of alloy resting on the bed. This would give 3 way indexing on a 3 jaw or 4 way on a 4 jaw.. With a bit of thought you could probably double that up to 6 way with a 3 jaw.
Alternatively, if your chuck is on a backplate, a series of holes in it's periphery and a peg to engage them, should provide lots of other options.
Bill