Can't you power everything with 3V3, including the Arduino? (In this case, the safer frequency would be 8MHz.) So you can get rid of the level shifters.
Impractical in this case - there are some irreplaceable components that require 5v - there are even some 12v signal levels flying around. I'm also using almost every single pin on a Mega2650 - so there are speed concerns. Thanks for reminding me to redo the power systems, though - I've got truckloads too much current on the internal voltage regulators.
20 feet is too far for reliable I2C although I know people do it.
Search for P82B715 and PCA9600 for a start.
Thanks for the tip. Phillips says on the datasheets that the standard bus is good for "a few meters" so I think I'm probably covered on distance. 20ft is the pathological worst case - the 5-10ft range is far more likely. I should also get some more range out by using the twisted-pair cabling effectively.
I've dropped a P82B715 onto the schematic as an add-on option - I'm hesitant to jump straight into it because you need one at both ends, and they're a bit expensive for what I'd like on the sensor boards. I'll test reliability-over-distance when I get to the breadboarding stage and make sure.