Even though the later part is just about sorting the partial keys, I included it for context. But the summary is in Tim Ruffing's first answer: MuSig is only going to sign all of the signatures, or none at all. There is no "backout" that the MuSig provides.

i.e. an open problem left open. Although as I mentioned above, garlonicon suggested I use Pedersen commitments along with Multi/MuSig. After reading about that, I am not so sure what the secret r-value should be that will, at the same time, be an invalid secret for an ECDSA or Schnorr-type transaction.