Simon,
I changed my seqn field to Supp Cat 3, a field I won't be using any time soon.
You need to be sure that your initial Arrangement order in source folder is the one you will keep during editing. Although you can set a seqn on that folder to preserve the order in the destination folder, you can not then re-arrange the images. If you click Arrangement, the sort order defaults to Filename, negating the purpose of your custom sort order.
In my case, once I have star or color rated my final selects, I will rename them, apply IPTC info, run them through LightRoom and PhotoShop for finished 8 bit tiffs. Now I no longer need the seqn numbers unless I need to match up a finished tiff with its raw original.
Paul