Another idea is the carradice sqr slim bag. My husband commutes on his carbon fiber trek, but keeps a spare seatpost/saddle with the bag clamp permanently mounted, and swaps it out when using the bag.

I commute with one too. Its large enough to accomodate a laptop, lunch, and a little bit more if necessary, although I use it on my steel touring bike w aluminum seatpost.

To answer your original question, I do know people that use those clamps just fine to tour with their steel bikes (i.e. ritchey break a way which also lacks eyelets for a rear rack). Tubus also makes some racks that mount to the same point as the brake, so maybe one of those would fit your bike.