Overall installing cables above underfloor heating is not a good idea. Installing them in ducting under the heat mat and under the heat mat floor insulation is a much better idea. However this does not seem to be an option in your case.
The maximum surface temperature for underfloor heating is about 35°C so a cable should be able to withstand that indefinitely, you would just need to derate the cable for the ambient temperature, however removing some of the floor to get the cables closer to the heat source does not sound good; would it be possible to overlay another floor surface and allow ventilation to the cables.