I finally found some time to post a HOWTO for the cross-compilation of MySQL 5.5.9 (latest version as of today).
The full text of the HOWTO and packaged DroboApp can be found here: http://www.droboports.com/app-repository/mysql-5-5-9
Comments and feedback are welcome.
Disclaimer: MySQL is huge. Just after starting, it takes about 270 MB of memory, so don’t expect it to have amazing performance.