Posting an answer to my own question. It turns out that this was simply a failed electronic throttle module (ETM), a common failure on 2001 V70 models. The original unit had failed. After replacement, the problems have disappeared.

Car : 2001 V70 XC, 127K miles, USA spec. Symptoms : When coming to a stop, the car surges ahead slightly just before or right at the stop ... engine revs a bit, and the car bucks forward an inch or two. Not in a dangerous-going-to-jump-into-the-intersection kind of way, but in a that's-really-annoy...