DV,
There is no hard guarantee that the crossfeed valves will remain closed in every future flight, due to human factors. Somebody will probably make a mistake one day. But, on that exact same flight when the mistake occurs, there has to be a fuel leak onto the crossfeed pipe for that mistake to be serious. This would have to be a new fuel leak, that starts during that flight, because the maintenance schedules require 100% examination of all fuel pipe couplings in the bomb bay and rear pannier before and after every flight.
Flight safety is definitely not compromised by not guarding the switch.
Incidentally, the only possible mistake that could be made in flight is to leave the switch at AIR ASSIST after ground starts, and a guard will not stop that mistake. The switch incorporates a positive action mechanism, which means that it cannot be accidentally brushed from one position to the other. It is effectively self-guarding against accidental operation. The officer who recommended the mod is a ground engineer because the incident with XV249 occured during ground maintenance. Had he been a flight engineer (and I'm not suggesting that the investigating offcer shoud have been an FE), I know that the thought of guarding the switch would not have entered his mind.
If the groundcrew want to protect the switch during engine runs, thats their business, not mine. I know that they start and stop the engines more than once during the runs and if they have an agreed opinion that proposes a guard, then they should start the process. However, they have a solid checklist that prevents the switch from being left open, so I would be very surprised if they want the guard.
Finally, that incident gave rise to RPM limits during engine ground starts to ensure that whenever the crossfeed pipe is open it cannot get hot enough to ignite any fluid (that should not be there anyway). This is a sensible precaution because it mitigates against a single failure during normal operation. The single failure being a fluid leak onto a hot crossfeed pipe during engine ground starts.
Ed Sett