Yes, just run it like any other amp in 4CM.
Guitar > Helix input
Helix loop send > Mustang input
Mustang loop send > Helix loop return
Helix output > Mustang loop return
This will allow you to place Helix effects before and after the Mustang, and also use the power amp and speaker in the Mustang to monitor through (in mono, obviously). Be sure to have an effects loop block active in the Helix (that corresponds with whichever of the four Helix loops you inserted the Mustang through), with the mix set to 100% wet.