I think NR only owns part of the formation. (Re-)Adding in the point end would then effectively be 'only' needing a private siding agreement, same as any other heritage railway connected to the national network.
I think the whole running line to Meldon Quarry was transferred to Network Rail with the sidings at Meldon retained by the quarry company.
I think the Okehampton Line is now classified as a passenger line to the stop blocks and is operated as one train in operation from Crediton. Im not sure if this is being done with axle counters or a line staff has been retained but any private siding point would need locking and possibly position indication to the signalling centre.
The hourly service to Exeter is going to make any movements onto or off the network rail section impossible during the hours of operation. I can't see that you could possibly justify the cost.
Most other heritage connections are from sidings and are not signalled form regular passenger use. Off the top of my head I can only think of the NYMR, West Somerset, Severn Valley and possibly East Lancs that are.
The majority are for occasion use and are connected through sidings. Ours at the Rother Valley railway is connected through a P Way siding. The Bluebell is through carriage sidings.
The closest example that i can think of to the situation at Okehampton is the North Norfolk Railway at Sheringham where it is an occasional connection that can be used up to 12 times a year.