HA! It turns out that my valve cover gasket is fine, I just had injector return lines that had turned to Swiss cheese. Brittle, frangible Swiss cheese.
There's a drain hole at the back of the injector cover, right aft of the crankcase ventilation PCV. The copious amounts of diesel leaking from the injectors (I sucked 8 ounces out of the injector wells with my MityVac) was pouring out of this hole, run ing down the side of the head and onto the hot exhaust manifold, where it vaporized.

The fuel running down the side of the engine made it look like oil was leaking from the valve cover gasket. I also had noticed that it smoked more when I drove aggressively. From this I had assumed that the presence and quantity of smoke was related to exhaust gas temperature (and that the leak was constant), rather than increased fuel flow through the injector lines when I opened the throttle.

Problem solved! Hopefully this will help someone in the future.

Symptoms: volatile-smelling smoke/vapor from under hood, fuel smell in AC, rough idle when started cold, loud acceleration, rough/jumpy running in reverse, soggy/unresponsive throttle
Solution: replace decaying injector return lines damaged by biodiesel

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
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1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)