How to modify ssh_args in rsnapshot config?

The rsnapshot man page seems to imply that you add the per-backup-point arguments as a comma separated list after the destination, the example they give is

backup /var/ localhost/ one_fs=1

This is the same as the other examples, but notice the fourth column. This is how you specify per-backup-point options to over-ride global settings. This extra parameter can take several options, separated by commas.

so I guess you would use

backup /var/ localhost/ ssh_args=-p456