Gerald Miller wrote:

IIRC there was an article in Home Shop Machinist many years ago
where the author had built a grinder head with a collet to grip
stub shafts carrying the individual wheels.

Indeed there was, used 5C collet. I built one myself using the spindle
out of a cheapo 5C spin indexer. Forget the bearings used, but by happy
chance, the OD was a slight press fit into standard bearings. I machined
an aluminum housing for the bearings.
I use this for wire wheels, scotchbrite, and nylon abrasive bristle
wheels, haven't gotten around to trying it with grinding wheels, but it
ought to work fine.

Jon