I recently put one of the old steel-clamp MTB shifters on a set of road bars with a 25.4mm clamp diameter. I assume the clamp for the shifter was originally designed for 22.2mm bars (or whatever the MTB standard is), but with a bit of re-bending and dig in the bolts bin for the correct length bolt, it worked just fine. It was an 8-speed shifter (Shimano, from the early '90s I believe), but I had a 9 speed drivetrain, so I just flipped it to friction mode and it worked fine.