If there were to be a "piping" leak in the grout curtain above 750ft to 922ft level, there should be leakage sign in the zone 3 fill along the seam of the hillside. There are ways to check for this using thin probe sensors. It would be easiest to do this with a test well in the rock & not in the zone fill dam materials.
Yes, the Grout Gallery Tunnels are fully functional for inspections all of the way into the tunnel systems within the Concrete Core Block. It is quite an intricate system. In fact, fans deliver fresh air into the tunnels from the upper Gallery entrances.
To extract these details requires cross referencing many documents, better imaged blueprints, reconstructing fragmented writing, comparing information & photos from inspection reports, etc. It takes a bit of effort.
Grout Gallery Tunnels in Oroville Dam & Concrete Core Block Gallery tunnels.
I was confusing the 'Left Gallery Access' entrance at 820' (normally not shown in photographs) with the entrance much lower and towards the center at 700' or so, which I thought was labeled something like 'inspection tunnel' or 'access tunnel'. I know the Palermo Outlet Works is/was also around that elevation as well, but (if I understand correctly) has since been closed off aside from a drain to Tunnel #2.
I'm reading through this fascinating thread an have only just reached the 2000's messages, so sorry for missing your post at 3450 - I'm getting there.