To follow up on this, I have done some further playing around.
Downloaded http://hg.mozilla.org/releases/mozil...9050ad.tar.bz2 into a microb-engine-stock folder in my Maemo dev VM.
Downloaded http://repository.maemo.org/pool/mae....2.orig.tar.gz and http://repository.maemo.org/pool/mae...%2b0m5.diff.gz into a microb-engine-maemo folder
A diff of both folders shows the only differences being changes to the .hg_archival file and the added debian directory on the maemo tree.
So that confirms that other than the patches in the debian folder, the maemo microb-engine tree (prior to any CSSU changes that is) matches to that specific revision of the Mozilla tree with no missing files or other changes.

Now I have duplicated microb-engine-maemo to microb-engine-built and am doing a build in that tree. Then I will compare the result (with applied patches) to the untouched microb-engine-maemo folder and see what's different.