Tom,
Unfortunately this response is not going to be much help for you however for the group, we have solved this problem a few times by insulating the slab under the in-floor radiant heat with one of our multi ceramic insulations. Here is a link to some pictures of one of the projects.
ie. NOTE : In this particular application the SUNSHIELD 2000 is completely buried in concrete and consequently only 2 of the 4 ceramics will come into play. The 2 reflective ceramics are ineffective in this type of application because the coating is facing direct contact, low temperature, front side conduction. This will reduce the R value equivalency to approximately R10. Additional R value equivalency can be achieved by adding more thickness.
An addition 7 mils DFT will only equal an additional RE3.
Prior to the application on the first project I worked with the Engineering Firm involved and we completed a successful test of this method of insulation using thermocouples buried in the test slab as well as taking top and underslab temperature readings.
Hopefully this information will be helpful to others that are faced with the lack of space constraint.
Best Regards, Doug