these are my two most unusual ones. i don't know what you need though
a total cobble together. the disc is off an old ten speed from when they were putting rear disc brakes on them (70's or so?). the wheel is actually a rear wheel that takes a spin on sprocket, turned around with the disc spun on instead. it's really too wide, but i forced it bettween the forks anyway. truthfully, it's not real strong, but together with the rear brake i stop ok.
this is off a bike called a murray street machine. it has attatchment points for two cables, so you can use two levers on the rear. it works really well. will lock the wheel up if you want.
couple others. again, don't know what you need.
just a 20" wheel on a 26" fork. the brake is on a cut out section of fork to get in in position to work with the smaller wheel. done this a few times. seems to work good.
didn't really have a more detailed pic of either bike, but it's the front end of one of those x-games moto x bikes. worth picking up the bike for if you can get it cheap. brake works pretty good.