Relief. stringing problem 100% solved by using Slic3r, a new gcode generation program. It’s also about 5x faster and generates gcode that prints faster, and is far easier to configure even than SFACT, which is far easier to configure than Skeinforge.
(Edit from 6 weeks later) In hindsight, the problem here was simply not using the proper retract settings, but SFACT hides retraction behind a wall and you can’t really edit it properly. The OldDimension module might fix that, but in the end I just gave up and went to Skeinforge)
There’s still some unevenness in vertical surfaces, probably caused by slightly low flow rate (I could have overstated the filament diameter, for instance) or perhaps the hotend temperature was a bit high. I’ll work on that.