It looks like I will have to do some serious testing with IR sensors to determine their sensitivity to scatter as well as reflection. For example, the Parallax BOE-BOT manual has a section that explains how to detect drop offs like the edge of a table using IR sensors. They show this figure-
If we were dealing strictly with light rays the light would be reflected off the surface at the same angle that it impacted it, and no light would bounce back to be sensed. However, no surface is a perfect reflector, not even a mirror, so some of the light is scattered and a part of it does end up traveling back to be detected by the sensor.
I thought, incorrectly, that the scattered light would be small enough that it would be very hard to detect and perhaps even lost in the noise. What I didn't know was that the IR emitters are pulsed at 38.5k and the sensors are looking for that frequency. This helps deal with interference and noise issues, and also implies that the back scattered light is easier to detect.
In a controlled environment, like the micromouse maze, the light patterns, especially the back scatter patterns, should be fairly repeatable and predictable. It may turn out that some experimentation with the sensors will yield a method to determine the wall status for the next cell before entry.