I don't use rsnapshot but you can set up arguments you pass to ssh, in which case you can selectively disable ControlMaster. If you can't, maybe rsnapshot respects .ssh/config so you can set up a Host entry with the relevant config. If you want to also use ControlMaster the host you connect to with rsnapshot, you can set up a Host with a dummy name and set its Hostname option to the real host.