Is there any compelling reason to remove the MP accounts prior to consolidating on a platform? They are already dealing with in-service limitations and QJSA requirements on those accounts, I would think that would become easier on a platform unless the provider cannot handle or handle differently than other portions.
Note that MP in-service can be lowered to 59 1/2 now too, if that helps.
If you really had to parse those out, I think you could spin-off those accounts into a new separate MP plan - essentially reverse the prior merger - and then terminate that plan. Participants would have to waive annuities with spousal consent, but you couldn't force that, and they could roll lump sums as desired into their IRAs or into the PSP.