Plenty, even with M10 studs. The double mount might have to be made in one piece for ease of assembly. It seem like an odd ambition in 2017, since rim brakes are a niche product for skinny-tyred road bikes these days.
So I've got:
Get double mounts designed made, brazed, re powder-coat, swap fork for straggler fork, enjoy 650bizzle
OR
Sell cross-check, buy straggler, enjoy 650bizzle.
Anyone ever seen an example of a bike with both 700c and 650b brake studs (they'd have to be removable, obviously). Is the 19mm difference enough?