This is somewhat verbose, but it works:
. {counter2:inum}Here is the first item.
. {counter2:inum}This is the second one.
. [[thisone,{counter:inum}]]This is the referenced item.
. And a final item.
Which one is xref:thisone[{inum}]?
For some reason, I had to use the xref:[] form of cross-reference to make it work.