There is already a “IPNI + POWO” taxon link set up for plants. The two data sets are now so closely linked that it doesn’t make sense to make separate links for them. The initial IPNI query result includes links to POWO.
The taxon links are just informational look-up links on the About tabs. “Search External Authorities” is a separate (and still working) facility for adding taxa not yet in iNaturalist, and I have proposed adding POWO to the available sources for that.
Yep, the taxon links definitely don’t address the main part of your request to store personal bookmarks on-site, which I also support. Just wanted to mention an alternate way to make links available site-wide for resources that have broad application and interest.