I know this doesn't directly relate to the BR pass but similar circumstances as discussed about being allowed to travel on Grand Central services... I often see SWR employees travelling on Cross Country/ GWR (and vice versa) on SWR routes. For example, I remember at Basingstoke a XC service was going towards Southampton/Bournemouth (SWR network) and a SWR employees waved at the TM and gestured towards the train indicating that she wanted to get on and the TM gave the thumbs up.
Even though technically when you get a TOC pass, you get free travel on the TOC trains rather than a network, (i.e. if another TOC operates in that area, you don't automatically get free train travel on it), in reality it seems that they have a casual agreement with one another. Obviously, however, if you are going off your network (e.g. if in the above example the train was going to Reading- off SWR network) then you are being extremely cheeky.
Having a BR pass means I don't have to do this but I'll admit this casual agreement seems nice as it means you don't have to wait another 20 minutes just for a train operated by your TOC to come to go to the exact same destination. Also, you get more choice of trains- not that I would rather be sitting in the voyager in the above example