All the (reputable) local machine shops are backed up 4-6 weeks so with a small amount of hope I decided to try and clean up the carbon from the valves & seats and lightly - lightly - lap the valves to see if they would seal. To save time in case it didn't work, I went straight to the worst chamber (middle cylinder on back head) which leaked water from every valve - prodigiously so from the exhaust side.
Each exhaust valve had a fair amount of build-up and the sealing edge looked a bit pitted - a bad sign for shade-tree mechanic sealing methods:
The combustion chamber before - intake seats didn't look too bad - I was confident they would seal with a little elbow grease:
Intake valve looked decent - only a couple rough spots:
The intake valves after soaking in solvent & a little elbow grease. Still a few rough spots, but probably lappable:
I forgot to take after pictures of the exhaust valves, but the sealing edge was still quite pitted. Lapping helped a little, but it really looked like they were going to need a proper grind. Anxious to test them, I popped them back in to see if they would hold water:
They barely weeped, so I'll still have to have a machine shop to a proper job. I hope if I clean & prep the head and bring it to them disassembled they might be able to get it back to me in a week.
The good news is the 4 valves I checked were within spec for length, stem thickness, etc. I don't have a tool to check the valve guides, but I'd feel comfortable running these valves in those guides for another 100,000 miles.