Road mechs are optimal for trials bikes. I used a Shimano Tiagra. It was OK. Some people used Dura Ace mechs some years back. Expensive but pretty and light. So yeah, short cage road mechs are great. The cassette cogs are so small, and in combination with only using a single small sprocket up front, you can get away with a short cage mech. You can lock it out too by removing the spring around the bolt that mounts it on the derailleur hanger, which prevents the mech from banging on the frame, while still shifting gears well. Removing the spring on a Tiagra mech (probably very similar to all other ones) is done by first removing the C-clip on the back of the main bolt (the one that mounts the mech to the hanger). then remove the spring washer thingy, and then remove the spring. Bolt the mech to the hanger with the spring washer thingy on, but not the C-clip. Voila.