Don’t really have an answer for you - but have you seen this topic?
[url]Linking Bookends entries and pdfs in Devonthink]
Here is the related discussion over on the Bookends forum:
https://www.sonnysoftware.com/phpBB3/viewtopic.php?f=6&t=3454&p=16967#p16967
It certainly worked at the time - but haven’t used it as much as I initially suspected, given that it served to overwrite any automatic links created when using the “Annotation” note feature in DTPO, the latter which I use extensively.
Regardless, the above might give you some ideas? Best of luck - shout if you find something! 