I know this has been discussed before, but I can't find the thread. I'm also aware of Rinard's Excel file, but I'd like to know if there's a general sort of rule of thumb. Generally, how does one accommodate spoke length differences for an OCR- or VSR-type rim?

Isn't it just like changing the flange width? The OCR rim moves the spokes about 2.5 mm to the non-drive side, so the effective flange width is 2.5mm bigger on the drive side and 2.5 mm smaller on the non-drive side.

It looks like the spocalc webpage includes the math...
http://www.damonrinard.com/spocalc.htm

Yes, it is just like adjusting the center-flange distances, but you'll find that this has almost no effect on the spoke lenghts. Try it out and you should get the same spoke lengths as if the rim were not offset (after the rounding to the nearest available spoke length of course)

Neil