They will actually let you use those as a component in the SRP/CS? I have a very similar application and everyone I talk to in the US says that is not allowable in my case based on requirements from 13849. There is no good way to monitor that the PO checks are really doing what they are supposed to. so we can easily make things redundant but we can't monitor them. In my case my risk assessment ended up indicating a Category 3 PLd design was necessary. If you can get down to a single channel non-monitored category the PO checks might be OK.
I am in the process of also looking at the Nexen NexSafe stuff, both free-standing rod locks and profiled rail brakes. I have also looked into the Sitema Safety Catcher and Safety Brake products, which are also free-standing unidirectional rod locks. They are pricey but they look pretty cool.
From what I can tell, to do this "right" you may need to crack out the checkbook and be prepared to add a bunch of zeros. Knowing what I know now I would have definitely steered clear of pneumatics.
Keith