The way we found to do this was with a subsearch and return $field .
get your sorting criteria (we ran stats on a subset of the source data)
sort the results
use fields to pull only the field used for your series
mvcombine to turn all the records into a single one
then eval with mvjoin to make it into a comma or space-delimited string
output with return $<your list field> (the $ returns only the field's value)
Probably a more efficient way out there, but this seems to work.
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