I don't see any way FI could differentiate between prime and sub-prime in a way that wouldn't incite considerable public anger, and quite probably lawsuits.
One back door FI could use would be to increase the impact credit limits have on FICO scores...i.e., make it so low credit limit accounts don't help as much as high-limit accounts. Since most low-limit accounts are subprime, that would achieve the desired effect without explicit labels being attached to other firms' goods that could constitute disparagement.
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in a credit-scoring postnuclear Stone Age...