Bioequivalence and Bioavailability Forum

Dear Dave,

I have seen such studies in the wild quite recently, targeted to EMA, where only a subgroup of subjects samples were analyzed for a second analyte.
Regulatory acceptance: Duno exactly. At least no deficiency letters w.r.t to this fact up to now. But usually only very rare feedback is given from sponsors back to CRO w.r.t. regulatory decisions.

To be on the save side consider going for a scientific advice beforehand.