Specifically, I'd like to do my keywording and categories in Lightroom for all jpgs and nef files (mainly because that is such a good app for doing exactly that), generate an xmp output and then get those keywords into the xmp / iptc fields.

Thanks for the pointer. Presumably I can isolate the fields I want? It seems that some apps use IPTC:keywords fields and others use the xmp-dc:subject so Ideally I'd like to copy to both although I can see that exiftoolGUI has a copy function from xmp to iptc and visa versa

In my initial tests with exiftool I noticed that copying xmp:all is pretty fatal to my nef files.

eg. I used

exiftool -TagsFromFile dsc_2774.jpg dsc_2774.nef or exiftool -tagsFromFile a.jpg -XMP:All a.nef

as a test (although I've not got to the bottom of why), I want to avoid complete xmp copies for now.

Recently I've been shooting JPG+NEF so I've been doing my keywording on the jpgs and then copying to the nefs, however I may only shoot raw on occasions so the eventual goal is to be able to do all keywording in the least time consuming way and burn to the file to avoid database dependency. Nikon software is totally useless for this job so I've only just begun to try to find a solution.

I shall have a play with Lightroom and it's xmp output, thanks for the tip.