I have a couple of these but I fear it would take many holes to find and then get a good view of all the fittings let alone making good all the holes invisibly afterwards.. . . just another thought: Could you use a smartphone borescope and just drill a few holes rather than taking the house to bits. Other thing might be pressure testing?
Many of the hot water pipes have their insulation pipes overlapping the fittings and would thus severely restrict viewing angles.
Furthermore, there is a lot going on within the underfloor space: not only the central heating pipes but also the pipes for the hot water system (including secondary circulation ones), and the cold water systems for both hard and soft water. Then there all the cables for the power, lighting, alarm and structured cabling. It's very busy in there and not conducive to waggling a borescope probe!