Firstly, I’m not familiar with RIS export at all. But I was wondering if a manual work-around is possible, pending @andreas 's feedback.
Here’s what I might try - (1) sync the paperpile pdf folder to a local folder, using Google Drive Sync on a windows system ; (2) export the desired ref as a RIS file ; (3) open the RIS file with a text editor ; (4) replace the portion of the paths in L1 fields with the sync paperpile location, using just simple text-based find-and-replace (For examples, \All Papers\ … replace with xxx\xxx\All Papers…) . I haven’t tried it, but just wondering theoretically if it might work.