Its common in a warm deck flat roof for example to place PIR foam boards directly onto roof joists/firings and then place a thin ply cover over with a flat roofing material bonded / nailed on top. That will withstand foot traffic easily (not to mention half a tonne of felt and ply).
In fact:
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"IKO enertherm ALU is a foil-faced insulation board and achieves a declared compressive strength of 175 kPa. IKO enertherm MG has a mineral-coated glass fibre facing and achieves a declared compressive strength of 150 kPa. This means they are capable of bearing static, evenly distributed loads of approximately 17.5 tonnes/m2 and 15 tonnes/m2 respectively without failure."
So say your fink trusses have 35mm wide top surfaces, and you have foam under 18mm loft grade chipboard panels. Standing directly over a truss, you load will be spread over say 400mm of truss. Or 0.4 x 0.035 =
0.014m^2. At 15t/m^2 load capacity that would allow up to 15 * 0.014 = 210kg of applied load.