Hello,
Currently there is no way to do clear DF bit on transit user packets on MX platform, be it with MSDPC or without.
To solve your particular problem, you can run MPLS over GRE across said link, and clear DF bit on GRE packets after encapsulation.
GRE reassembly must be done by MSDPC.
Do not foget to enable PMTUD on GRE tunnel as well, and use MSDPC to adjust TCP MSS.
TCP MSS adjust must be done before GRE encapsulation and/or after decapsulation.
HTH
Rgds
Alex