Bad news James, with it all on one head I would be looking at oil starvation.....
Yep, sounds like it to me.
Assuming this is correct, and given James's meticulous approach to his work, this is extremely unfortunate, and leads me to ask, what more could he have done to ensure that there would be no interruption to the oil feed to the camshaft bearing(s)?

Fitting the head gasket wrongly is the most likely cause that would affect just one head, IMHO. The feed is staggered, so relies on an elongated hole in the head gasket to transfer supply from block to head. It
might be possible to block that feed by fitting the gasket incorrectly.
Failing that, maybe the oft-encountered problems with blocked pickups in the sump on a 2.6 prevented the oil pump from priming correctly. One would expect that to be immediately evident in knocking from the crank bearings, though.
I'm not sure if the bearing oil feeds pass through the T vent on the head. Possibly, this blocked due to some dirt that got dislodged during the rebuild?

I'm sure James will carry out a detailed post-mortem in time. Every cloud has a silver lining, good excuse for an upgrade to a 3.0.
