It sounds like what you are doing is "in-filling" and encapsulating the pipe ? Correct. I would think you need a certain thickness over the top, depending on the material being used.
I think when it comes to the materials you propose to use, you really need to pay strict attention to specifications of such materials.
The Gypcrete I attached a link to below shows a compressive strength of up to 3200 psi, but at 3/4" thick-7.2lbs/sq.ft.
The problem I see with this material is 2 things, one being I firmly believe you will not achieve the same compressive strength at 2-1/2". The material is designed as an underlayment, if you form it per your job as described, place same, I do not believe it will achieve any strength by virtue of design. The other is being gypsum based, in many instances gypsum based materials are not as strong, another example where its true is spray applied fireproofing, for structural steel.
Maxxon and I have used their underlayment products on a large scale, does appear to offer a material that will achieve a satisfactory compressive strength, and that is thick enough to embed or encapsulate pex, and is designed for the purpose,(link attached). But it can only infill 1-1/4", so maybe that and an underlayment layer to make height?
Maxxon has a bunch of products elsewhere on their website, you have to look at dry density for weight calculating, compressive strength/thickness, and point load,to insure you have the strength you need at the thickness's required, as well as dry density weight for loading, it tells you per square foot what it weighs and thicker/stronger is definitely heavier.
You could use a 1000 psi light weight small aggregate concrete, or maybe 2000 psi if they offer it, them use and underlayment if your not the best finisher, leave enough room for the self leveling underlayment, it will leave a nice level-flat surface.
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