I like to back up /etc folder and have always had to backup via the file system backup module to local disk and then rsync to remote server on non standard ssh port.
I know many have complained about the file system backup module not allowing ssh port other than 22, and due to the way its coded I realize it can not be supported. So, A possible work around may be to use the webmin, “backup configuration files” module instead? It has a check box to include “other files”. I included /etc and it backed up the entire /etc folder along with the webmin config info via ssh over port 225. Although part of /etc is backup by default, adding /etc to the other files list, backs up entire /etc folder.
Looks like any folder or file name could be included in the “other” box. Handy for those not ssh’ing on port 22.