Good stuff Russel. Nicely done.
But beware. Once you stitch you can't stop.
One way to go around loading every single sub in RAM in registar is to 'batch'. You first load the sub that is sort of in the middle of the finished product. Then register the whole directory to it as union. If you're short on RAM you can do this one at a time and save the intermediate file. Once you have your scaffold, load it then register all the subs one at a time from the original directory as union then save each file and unload it from RAM. This will allow you to mosaic large areas with minimum fuss and RAM. If this is not clear just PM me.