Enemas have gone better than expected. After trying a few things we have settled on something very similar to what we were already doing. Basically, we now tape trash bags over his casts after we take the ace bandage off, to keep them dry. He lays on the floor on his left side with some towels bracing him and the orange cast. We use the cone enema to put the fluid in, then we sit him up (he's on a diaper and towel) and quickly lift him onto the toilet. We do have a child potty seat insert on the toilet. I sit in front of him on a footstool and hold him because the heavy casts wreak havoc with his balance. We sing silly songs, watch silly videos, and do anything that makes him laugh. Laughter + Enema solution = Poop.
He is still taking an increased dose of Miralax daily to keep his stool very very soft, just this side of diarrhea. We are generally avoiding the main constipating foods (cheese/rice/bananas/etc). We're also encouraging Mason to drink lots of fluids. He won't drink juice but we've found he will usually eat juice if we freeze it into a popsicle, so that is a regular snack each day too.
Oh, and we're doing a lot of dancing right now. He's loving it!