Thanks. Unfortunately, no one has the NVRAM in stock! I don't want to spend $4 shipping $10 of batteries.
So this will be a bit tedious. But I bought two SD-22s for $150 and a 1 m extension cable for $100, so with bit of luck and work I'll have a 4 channel 12.5 GHz sampler for under $500. Now if I can just find an SD-24 and a SD-30 or 32 for $1000 total. The cable gives me a shot at fixing a sampling head if one has a simple fault.
How should I do the copy of the NVRAM? I was thinking of powering it up for a day or two, shutting down, pulling the chips and copying them as quickly as possible in the hope that the internal batteries are dying, but not so dead that the contents are gone. From the service manual it appears that the 2/3 A lithium cells are backup for the NVRAM batteries.