Indeed, even adding basic 'switch out' functionality is complex and expensive, and comes with restrictions.
Switch out/through working logic is known and doable, but without track circuiting or axle counters reduces capacity, and of course flexibility.
As previous, my understanding is that the line is already track-circited, as it is worked by direction lever.
A basic switch out functionality need not be complex, and shouldn't be expensive if the work is done by volunteers.
Before closing out, the Bewdley and Kidderminster signalmen set the route through from NR to Bewdley. The Kidderminster signalman then extracts the special Kidderminster NR to Bewdley train staff, which electrically locks that route in, e.g. by back locking the relevant signal levers. To allow the train to get back, signals reading in the opposite direction have an indicator that is lit allowing them to be passed at danger if in possession of the special staff. No mechanical locking alterations required, just some fairly basic electrical controls that it should be possible to rig up fairly quickly.
The Kidderminster signalman then puts the train staff for incoming trains off NR, in a locked box somewhere convenient. I am thinking of something like Ambergate, where trains going to Matlock collect the token from a locked box on the platform.
I am not saying this is the ideal way of doing it, just that you don't need something complex.
I also can't see an automatic, permanently off slot getting any kind of approval or assurance because there are too many variables still at play - especially moving from the national network to private infrastructure.
There are plenty of instances where trains run from NR onto private infrastructure. I don't see what the problem is. I presume that there would need to be someone on duty on the SVR in case of problems, but you don't need a slot to prove that. For example, when a loco hauled train goes to Blaenau Ffestiniog, the signalman at Llanrwst phones up to check that the Ffestiniog Railway has a flagman on duty at the foot crossing. No need for anything more complex than a simple phone call to check that the SVR has someone on duty, and is happy to accept the train.