I've recently been thinking along similar lines albeit making use of a 4u rack case I have lying about and filling it with goodies for a "silent" stage rig. So far three spaces would be used with a sansamp RBI, tuner, and compressor, but then I would like an in ear system to complete the rack. But this is where I am faltering as what I would like to do is to be able to mix in my own bass level with whatever the desk is sending me so that I am not reliant on the PA operator to give me my own level. The closest I can find to this functionality is systems that take a L and R feed in, but then allow you to be in "mono" mode and mix the L and R to your own liking.
But I'd be interested to know if anyone knows of a specific IEM system that has it's own mixer built in specifically.