You are right, it is currently not possible to generate an installation link for Bitbucket.
If the goal is to try to install the app on a workspace that is administered by others (as a reminder only workspace admins can install apps), the only available option today is to:
ask the admin to install the app on the workspace via the Forge CLI
I believe this request might have been for Codegeist, our annual annual hackaton, for that case we provided a dedicated alternative (see 💰 $170K In Prizes - #CodegeistUnleashed is LIVE!).
Sry for not seeing and answering this message earlier! I hope you still managed to submit the app and good luck!