Huh. Didn't think this thread would make it through the board remodeling. Anyway...

Replaced the valve guides. Didn't do a damn thing. But when I replaced the thermostat a few days later, during the bleeding warmup I observed that the burning only happens when the engine is revved with no load, or shortly after idling/running with no load. Climb a hill and it's clean. Descend a hill, and it starts puffing.

I'm thinking the bottom end is bad, but I've been advised to check the turbo. I just got two spare engines and a total of four spare turbos (at least two of which are the later wastegate-type), so there's some hope left...