Is there any way to set the payment processing profile on SCA orders based on customer category or something? We want to use two different profiles: one for B2B and one for B2C.

There is not. I managed to make this via an scriptable cart but i wouldn't recommend going that path - feels extremely hacky

What about setting it at the customer level in general? I noticed there’s a Preferred CC Processor field on the customer record. but it doesn’t seem to actually do anything?

In my experience, it's ignored

Yeah, that’s what I’m getting too.

Why two profiles?