In case there's anyone like me for whom the Windows-only-ness of tools like PlasmaShop is a major nuisance, here's the script I'm using to regenerate the checksum file (using facilities from PyPRP). I should have a quickly-and-dirtily ported Python PAKker around somewhere too, if anyone wants it, but haven't used it for Pahts so far.
There is an even easier way to prevent the sum file from getting invalid: Just create a sum file which references only the BultIn file from Pahts (or any other file which rarely changes). All the other files don't have to be mentioned in there, so that changing them doesn't make the sum file invalid, so you don't have to constantly update it anymore. I could upload it here if the board wouldn't give an error while uploading zip files...
Oh, and btw, PlasmaShop works great in wine, I use it all day...