Yes, that's it. I use customized OpenJpeg (loading/saving CDEF boxes in JP2 files to get proper support for images with alpha channel) library. I submitted the patch but OpenJpeh team said it could be added to 2.0, not current 1.3 because it changes library interface (hence the record sizes mismatch).
Can you use the compiled OpenJpeg lib from Imaging release? I could send you the patch if you want to compile it yourself. But note that OprnJpeg compiled in GCC with some optimization options screws up data of fourth and subsequent color channels.