I never took you for Necro-phobe Sailor...

I would be interested in the science behind "curing" concrete slabs by warming them up with embedded radiant PEX. As the concrete curing process is already exothermic and the output of heat and moisture tied to the initial chemistry, I should think adding heat in any amount would be high science and beyond the skill-set and experience of most. You may use thermal imaging to monitor the "surface" of the slab, but factors like insulation, ambient temperature, wind speed all play a factor.

I am curious, but it looks like too many moving parts for this cowboy...

Exactly right Badger. While it is quite common to use piping to cool large concrete pours (e.g., dams), it is quite uncommon (although it can and is done) to use piping to add heat to small concrete pours. In all cases the enemy is creating temperature gradients across the pour during the curing process. One must always remove heat from large pours to avoid this situation. For small concrete pours typically found in commercial and residential buildings, there is less complexity and less risk to just accomplish the pour when the temps are above freezing. Logistics and planning go a long way to having a successful project.

Pump 65F water through it and it won't add any heat if the concrete is already above 65F. But it will speed curing and should prevent repeated freezing once the temperature drops. See "heating coils" here or see here.

There is not a single magic temp of water to pump into a concrete pour. You need to keep the temp gradient under about 35 deg F for typical concrete. To do this requires knowing the air and ground temps, the pipe spacing and location within the slab, the thickness of the slab, and the R-value of the below-slab insulation and temporary above-slab insulation blanket. Then you use an accepted analysis method to determine the temp gradient such as the Schmidt’s method to ensure that you won't actually exceed the temp gradient limit. The real problem with cold weather concrete pours is working the concrete.

Took time to discuss this with my P.Eng. this afternoon. Not only does he recommend this method but has done several in Calgary and one in northern Manitoba.
Again, not hot water but between 50 and 60 degrees.
Still a problem if you need to finish the a slab but for form work and sub slabs it works.
Concrete is very conductive of temps and wet concrete more so. Unless you have very thick concrete to deal with your are unlikely to get temperature gradients. That is why concrete cooling is used for projects like dams but almost never for average concrete work.