Mr @ Spotty M
I am no avionics engineer but l know you can remove the plug and check that when you test the system, the signal you are sending to the R/H head does actually arrive at that head and not the other head.
That's my recollection of the system as well. A simple test light can verify the cockpit to fire bottle wiring integrity. If Boeing can be bothered to spare two technicians and a couple of handheld radios. Likewise, proper routing of flexible Halon tubing can be verified with compressed air or dry nitrogen prior to installing squibs or burst discs. But that's a manufacturing step and the bean counters love eliminating those.
It might be the case that using keyed connectors is not possible due to the same model number fire bottle being used on too many different aircraft to retrofit a design change. However, I seem to recall that the layout of the electrical and halon plumbing on a fire bottle is such that the 'Left' plug is physically adjacent to the 'Left' Halon outlet. So fastening the plumbing and wiring in a simple clamp would prevent accidentally separating one system's connections.
A diagram of the bottles and 787 installation would help greatly.