It certainly appears that the NR signallers are being used by the RMT as a weapon for a battle with the TOCs.
The signalling grade do seem to be in the greatest position of strength. I wonder if the RMT could leverage that further.
Some very rough maths from (probably inaccurate) google suggests there may be roughly 5,000 signallers out of 40,000 strikers at a 1 in 8 ratio.
I'm not a lawyer and probably talking unworkable rubbish, but could the RMT keep signallers on strike whilst allowing everyone else to work. Temporarily up the union subs by say £50 a month, which could be used to pay each signaller £400 in strike pay a month indefinitely.
That could buy them a few days without trains each month for a £50 cost to each member. It would probably require re-balloting with very careful wording to incorporate all of the RMT's issues into the signallers ballot to make it legal etc, but could strengthen the RMT's position if this is going to turn into a drawn out saga.
It may also keep the signallers onside whilst fighting for other grades as the way to end such action would be to make the signallers in particular an offer they can't turn down.