[BScout]

Not the only way.
You can have the MTRX reference its clock from digilink or word clock too.
Pretty much, Pro Tools has no idea what the MTRX units are doing. And the MTRX units can be set to reference any clock you want.

And for frame edge, Pro Tools only needs to be "using" the SyncHD as the clock reference and then the Sync HD needs to reference your tri-level sync. So the "loop sync" part of it really doesn't matter. You could also feed the MTRX with word clock from the SyncHD, for instance.