Hm, I'm not aware of anything in the Pauline Law to cover such a situation.
This is pretty much what happened in France in 1316 and again in 1328. When Louis X died in 1316 he left daughters and a pregnant wife. His brother Philip became regent pending the birth. The child was a boy and technically reigtned for a matter of days as John I the Posthumous before dying (rumours of foul play). Philip then succeeded as Philip V in the normal way. He died in 1322, leaving only daughters, and was succeeded by the third brother as Charles IV. Charles then died in 1328 leaving daughters and a pregnant wife, and his male line cousin, Philip of Valois, became regent. The child was a girl and Valois succeeded as Philip VI.
When Alexander of Greece died in 1920, leaving a pregnant wife, there seems to have been no question of a regency as the marriage was treated as morganatic.
I don't see Mikhail as beeing all that ambitious. Had Nicholas died in autumn 1900, I think Mikhail would have been content to be regent had the child been a boy.
Ann
