I don't think Dispatch is going to work for you.
Without an add-in, the easiest way to update many files' variables at once is to [temporarily?] put the variable on the folder card and use Modify > Update > Values in files
Thank you the info - I was worried that was going to be the case. The users may me doing this quite regularly and the files are nearly always in different folders so I think an add-in is the only option.
I've looked through the documentation and the only variable update examples I can find look like this...
And I'm not actually sure that's what I'm looking for - do you know of any other add-in examples that could get me on the right track?