Here is something that will likely bring me a comment or two along the lines of why would you do that.
I finally found some images of doing this, so posting.
When I built our attached garage I put in a pit , and a downstairs room in the front of the shop.
If this was not enough I put in hydronic heating, not just in the floor but the walls of the pit, and that room!!
Of course had to line these blocks up while putting up the wall so all holes lined up close enough to do this,
and still be half of the rows above and below.
In the images here the pex was *only* touching on top
temporarily,
it was even tied together in places. This was to keep the PEX stable so it would be less likely to kink as the cement was poured and vibrated in. After a few feet I would go back with the cement vibrator and pull the PEX into its final position
As the PEX went down they were separated with rebar I welded up specifically made to hold the PEX and help guide it down without kinks.
If I can find THOSE images to show how I did that I will post them.
Excuse the mess there on the left, was a long day, and cement doesn't wait for anyone, so things got tossed as they were in the road
This all makes for a very nice place to work in the cold Mn winters.. You can lean against a wall changing oil or just working under something without freezing. It also seems to had an unexpected effect.. Normally in a pit like this when someone opens the outside doors you get a massive rush of cold into the pit. Not this setup! It seems the draft is significantly slowed, to that point its barely a hint of a cool breeze by the time it reaches you downstairs.