We came across one post that mentioned using a Somfy Centralis DC IB for controlling a Somfy DC blind motor. It seems to drive a motor in both directions based on up down and common inputs - seems to be just what we need. The manual didn't have enough information for us to be confident about this unit, but since it wasn't a hugely expensive component I decided to buy it and give it a go. It's available from Simply Automate for 28.42 GBP: Centralis DC IB.
We've also started thinking about using something other than X10 for some of the controls now. This was partly motivated by the number of X10 devices we would need to control the blinds. See Beanz's [future, ed.] post on the VIOM.