After some testing, it would seem that 6B>2A>5B>623B does not work on Mu-12, as the 5B lifts her too high for 623B to connect.
I don't know if it's just my execution, so if someone else could test this as well, that would be great.
Combos that do work on her:
*Also, none of these offer seals, so... yeah.
1. 6B>2A>5B>j.B>j.4C>j.623B>Land>5B>2B>j.4C>j.C [From Rei-Scarred, True Oki, 2,548 Dmg]
2. 6B>2A>5B>j.B>j.4C>j.623B>Land>5B>2B>j.B>j.4C>dj.B>dj>4C [No Oki, 2,735 Dmg]