Can I run PEX through PVC pipe embedded in a concrete floor with radiant heating?
I can see two things to be concerned with.

1) Is the 1-1/4" PVC pipe of sufficient size to permit you to pull out the old and replace new PEX tubing if that should ever be required in the future?

2) Make sure that the PVC pipe does not detract from the thickness of your slab to the extent that it would weaken the concrete and lead to a potential crack path. Cross wire mesh and re-bar in the concrete will help a lot but consider some additional thickness in areas where the pipe is embedded.