well Dirt map is still bit different from cavity map, for most render engines it give "sharpened edge", in other word it outline other side of edge then where the most of dirt is. For better understanding what I mean I suggest you to check next image, left is AO map, second is inverted Dirt map, they both had basicly same search distance and sample rate:
Before someone mention it, nope, if base color for AO would be grey, then result wouldn´t be anywhere near dirt map either, it wouldn´t give
In final render I wasn´t able to render cavity map (first version of workflow included Z-brush, which is able to do so, but in the end it was just overkill for this resolution). So in order to get Z-depth for details and even for whole I used Z-buffer pass, later combined with rest.
I really don´t want to flood this topic, and I would like to ask you to use my "dock":
http://fractalsoftworks.com/forum/index.php?topic=6373.0 for more questions and discussion about 3D to 2D technique.