After a bit of head banging, I managed to get MySQL 5.5.9 (latest version as of today) cross-compiled and packaged as a DroboApp for the DroboFS.
You can find it here: http://www.droboports.com/app-repository/mysql-5-5-9
Warning: out of the box MySQL takes about 270 MB of memory - which means it is already doing some paging. In other words, don’t expect this to be a speed monster. Also , it may impact on the overall performance of the FS, but at least it is available.