Well neither RAB or ME seem to currently do convertibles either, so I'd presume the market must be quite small.
Haglofs have one style, Rohan 3, Pattaguicci ditto and Fjallraven huge numbers! No doubt others too. Very few with the reinforcements that Terra's have, if some quite tough fabrics.
I've never been tempted mind, so no idea beyond Chris Townsend in some TGO review emphasising how important it is to get the fit right. (Rohan presumably easiest for checking that.). Might well be worth trying the standard Terra's though - the vent does help a little and they're really not very warm.
Edited: 14/06/2012 at 12:17