All you need is a locally controlled signal either side of the "level crossing" with a fixed distant braking distance from it. Not uncommon for older level crossings in the middle of block sections.
But those signals need to be tied into the signalling control panel, without disruption the existing signals - not a straightforwards undertaking. It was much simpler in the days of local signalling and mechanical controls - then the signalers just needed to learn a new working practice.
Unfortunately computer software is much less flexible than a human being - when it comes to dealing with a situation the designers never considered!
Sorry but I cannot see why they need to be linked in any
way to the RETB signalling or its' panel. They would not be signals demarcating a block section. They wouild purely be locally controlled signals to protect the "level crossing". All that is needed is a local control "gate box" and when the level crossing is clear of traffic, signals in both directions can be cleared at the same time (as only one train can be in the single line section at once this is not an issue).
As far as I can see, all that is needed is notices in the sectional appendix to say that locally controlled signals protecting the "level crossing" will be installed in the up direction at x miles x chains and in the down direction at y miles y chains to protect the level crossing, with fixed distant signal boards at xx miles xx chains on the up and yy miles yy chains on the down.
Then all I would have thought you need to do is go and install distant boards at the braking distance and 2 aspect colour light signals or semaphore signals either side of the crossing, clear it with the signal siting committee and arrange for any supplementary driver route knowledge training.
This sort of arrangement was common for level crossings in block sections and the signals were not linked to the block signals. All the gate box had were levers to pull off the signals and block bell repeaters so that they knew when a train was in section and they had to shut the gates and pull off the signals.
You would need some non safety critical radio or telephone contact with the RETB signalling centre (to replace the now non existent block bells) so that the signaller at the gate box knew when a train was approaching and could clear the crossing (or some sort of treadle system to alert the signaller a train was approaching), it wouldn't be safety critical as if a train turned up "unexpectedly" it would just be brought to a halt at the signal protecting the crossing.
However even this would cost a fair bit to do, and may well require some interlocking between the local signals and the level crossing flashing lights/barriers, TPWS etc, so it may well be that they were not prepared to pay the cost of this for a verty short time period.