On fitting 700C rims to a dynamo hub rated for up to 20" I am currently in the planning stages of creating a really fancy front fork for my bike.
For a little background, I want to try to get this custom built Burrows monoblade with disc mount:
https://bikefix.co.uk/blade
And attach a dynamo hub to it.
The thing is that I have so far been only able to find the Schmidt SON XS-M series of single-sided dynamo hubs which seem remotely correct, and all of them are listed as max 20" rims, while I want 700C rims.

My question is this: What would happen if I tried to fit the larger rims to the smaller hub, and why shouldn't I do it?
Generator hubs for small wheels will output less and produce less drag at a given speed when built to a 700, because they'll be operating at fewer RPMs than expected.

AFAIK all the non-sondelux Schmidts still have output suitable for halogen, including XS-M. You should double check when you order the hub. If so it will pair fine with LED in a larger wheel.