Not exactly the data point you were looking for, but I run nominal 700x40 WTB Nanos on my green Rambouillet. I use Paul Racer M brakes on that bike to achieve max clearance, so it may be similar to what you have on a Canti-rom. The tires measure out around 37.5mm on narrow Velocity Aerohead rims. The limiting point on the tire clearance is the chainstay bridge. I picked up a pair of Steilacooms at a bike swap last year and plan to replace the Nanos with them soon so I will have more specific data then.
Picture of the current setup:
-Jeremy Till
Sacramento, CA