In this message
BCLOUD-6705 , it is advised not to use bookmarks but instead the evolve extension for PR.
evolve works fine with the topic extension (both are included in the same package hg-evolve).
When the experimental evolve support is activated (https://bitbucket.org/account/admin/features/), topics can already be used with Bitbucket and we would just need to be able to visualize them in the website.
We would only need few very simple modifications of few pages (in particular PR creation, PR summary, commit list, commit summary).