I have to add this post because the procedure was driving me crazy. I am using WAIK 7 PE 3.0. I used the guidelines from https://www.sit.auckland.ac.nz/Create_a_custom_boot_image
The following workaround to the recommended worked. I tested it twice with i86 and x64 images. Used dism.exe and peimg.exe and imagex /export
I imported the necessary cabs into the wims as suggested in the guide. Ok.
I checked the packages. Ok.
I finalized the package. Ok.
I exported the wim. Ok. --------- Ooops, forgot to commit. But wait!!!
The export worked. Since the "recommended" procedure failed so often, I pressed forward and did not run the unmount/commit.
The boot.wim was accepted into SCCM SP2 R2, even though I did not unmount and commit.
I tested this twice and wanted to pass it along.
Ironically, when I unmount and commit after finalize and then attempt to load it into SCCM 2007, I receive an error.
I would like to see if this helps anyone else. It seems like a lucky accident.