One thing that might help in the future to do a global mod is to see what code NWC generates for what you want on the clipboard. Copy a note ... insert it ... change the attribute(s) you want. Select both to the clipboard and run a text editor with the copied text. Note the difference(s) and figure out a way to make just those changes globally.
That was pretty well what I tried to do and I suggest that it will work very well for every attribute that is represented explicitly in the text for both notes. In the case where one note doesn't mention the attribute (e.g. "|Note|Dur:4th|Pos:-3|Opts:Stem=Down" and "|Note|Dur:4th|Pos:-3") then I had to try a few variations to try to turn it off. It was interesting to see the slight variation of syntax to turn off 'ArticulationsOnStem' and Stem Direction. I'm almost tempted to embark on the catalogue of all attributes. Perhaps when the grass has stopped growing and that paint has dried properly...
The Global Mod tool appears to be just what I want (many thanks to Andrew) but I couldn't find out how to turn off some of the attributes that can be set.
In particular, I can set Stem=Down or Stem=Up but I can't see how to reset it to the default value. I know that I can do it pretty easily outside the user tool using Tools\Audit Note Stems but I wanted to be able to reset the stem direction at the same time as resetting XAccSpace, XNoteSpace, Ties and Slur directions (for which 'Default' is a valid option). I tried "Note,Pos>=0 Opts.Stem=Down Note,Pos<=-1 Opts.Stem=Up" but this didn't seem to behave as would expect (occasional notes left with an unexpected stem direction) and, anyway, I wanted to remove the explicit stem direction not just change it.
This example is the most important for my use but I also find that I can't turn off ArticulationsOnStem (use "Note Opts.ArticulationsOnStem=" to turn it on) or 'override stem length' (which appears to be set automatically if the stem length attribute is set explicitly).
I wondered whether there was a similar behaviour with other tick boxes and, although I didn't try them all, I found that the 'Preserve Width' attribute for text items can be turned on "Text Wide=Y" and off "Text Wide=N"