I just tried it, and indeed, you're right. (Although it works if you extrude the surfaces one at a time, but that's not too helpful if you have a lot of surfaces.)
Andy or someone else might know a better way, but in pinch you might be able to use the displacement mapper plugin as a work-around:
http://www.independentdeveloper.com/...acement-mapper
Duplicate your object, apply an all-white displacement map to the duplicate, then merge back together with the original if desired.