DV, the problem with your idea about monitoring the temperature of the closed crossfeed duct is that you are moving into a triple failure condition if the sole reason for a gauge is to avoid a fire.
In no particular order the following failures must occur for there to be a fire on the surface of the closed duct:
1. Hot air leakage into the duct across a crossfeed valve with its dedicated indicator showing SHUT. The leak must therefore be very slight, bordering on insignificant, unless you want to add a 4th failure (defective indicator).
2. Air leakage out of the duct through a failed NRV, loose clamp or simply a hole. There must be flow for the leaked air to raise the temperature of the pipe skin. Yes, this could be a pre-existing fault.
3. There must also be fuel on the pipe.
For the above reason, we do not need a temperature gauge if we want to stop a fire. A hot pipe gets dangerous only when fuel is in contact with it or is very, very close in vapour form.
I am aware that QQ suggested a gauge in their report, because the individual they spoke to about this aspect of their investigation was a flight engineer. He also added that, as an eng, he is interested in everything that can happen in the aircraft and that it would be useful to know if a closed valve had leakage, so that he could report it when he landed. Thats all very well, for the purpose of monitoring one system, but there has to be additional failures before that defect contributes to a fire leading to a crash. We do not mitigate against double or triple failures. We will not use up resources just to provide the air engineer with another "useful" gauge look at.
We maintain the aircraft to the highest standards, thus minimising the risk of single failures and we fly it likewise, thus preparing ourselves for the rare occasion when the single failure gets past maintenance.
XV 230 (and all the other aircraft) had a design flaw that negated the high standards that we operate to, on the ground and in the air. I recommend you and the other critics of the Nimrod examine the cause and contributory factors of that design flaw, rather than question the way we are moving forward now.
Incidentally, a single failed valve could be diagnosed in flight if a temp gauge was fitted and a (defective) point of exit also existed.
Regards
Ed Sett