I wrote a simple pop-up window that lets you quickly find the BPM for a song manually.
I use this all the time with Jazz library, which is incompatible with auto-BPM generators. The BPM value is saved to the file tag, which ensuring the value is permanently attached to file even outside of MusicBee
For more information and the download please follow the link to GitHub.
The plugin is open source so feel go ahead and open issues, send feature requests or customize it for your needs.
Addon: Coming soon