Yep. More-or-less what was done for our conservatory floating floor four years ago. Concrete beams with blocks between, covered with screed, and then 50 mm Celotex. Heating was electric, though - film laid on top of the Celotex, then 1000 gauge polythene on top of that. Actual floor was T & G laminate (conservatory has potted plants, so polythene was needed to ensure any water which accidently got through wouldn't get to the electrics).
You're reading into the max. temp thing way too much. Crank it up to 55 degrees and don't worry. The 27 degrees thing relates to the top surface you ultimately walk on not the surface you lay your top wood on. i.e. if wood says it can only be heated to a max of 27 degrees you need to apply significantly greater level of heat to achieve that temp. That's why wooden handles are used on things that get hot like... pans, pokers etc.
Before I had loft insulation after refubing the whole of the upstairs living area including ceilings, and while I was trying to decide what to do with the lights we had a particularly chilly spell and the UFH wouldn't bring the rooms up to anything like bearable as they were pretty much open to the cold of the night so I pushed the heating up to well over 60 degrees without any problems.
I have 50mm celotex between the joists, ufh pipes clipped to that, 18mm chip board floor fixed to the joists, brown paper and 18mm engineered floor on top of that.
The boiler is weather compensation regulated with a max. flow set to 50 degrees. all UFH manifolds are limited to 55 on their mixer stats as a "just in case".
Temperatures under rugs and mats will creep up a little but I doubt they'll get to 30 degrees. Typical temp. I see on the surface of our floor is never close to 27 degrees. Run it at proper UFH temperature (50 to 55 degrees) and wander around with an infrared thermometer if you're concerned. If it hits 30 degrees on open floor just wind the temp. back a few degrees but I'm sure you won't see anything like that as heat is always being drawn away to warm the room.
HTH Pete
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