Just for curiousity, the Boss SG-1 Slow Gear from the early 80s did automatic volume swells. I've seen other copies of this pedal since that did the same, as the Slow Gear was only manufactured for a few years and is both rare and expensive today. I believe the Slow Gear is the basis for Line6's interpretation of this effect.

And +1 – I would love to see something along these lines added to the ModFactor. I agree, too, that the MF is the more logical choice, since as stated, this is a dynamic/envelope effect that could be modulated, but isn't time/delay based itself. And, I could think of some really cool combinations of auto swells and delays, which likely require both the TF and another pedal (such as the MF).