The "rebuilder.sh" script at /usr/portage/dev-db/mysql/files/rebuilder.sh doesn't notice that QT may need to be recompiled because it doesn't scan through /usr/qt. This should probably be added, as my QT was linked against libmysql, so I imagine other people would run into this as well.
Created attachment 10097 [details, diff] rebuilder.sh.diff It seems a bit silly to post a one-line diff, but here it is anyway...
Fixed in portage - thanks!