The solution I've come to is using a second pivot table using the same MS Query (just copied the sheet containing the original pivot table).

I then have the second pivot table filtered to show the opposite of what I want (i.e. where Field1 = 0 and Field2 = 0). Leaving the first pivot table unfiltered, I subtract the second pivot from the first pivot to arrive at a derived Field1 = 1 or Field2 = 1 filter.

There are little things to be careful of (like setting the option to display field values that do not contain data), but it does the job.