SSH means CLI only
I’ve just tried this with sshfs, and it works just fine. It isn’t CLI only: once mounted, you can access the share from any software (GUI or CLI).
I found I needed to change/add some options in /system/settings/ssh/sshd_config (on the haiku box):
SubSystem sftp /system/lib/openssh/sftp-server
Also you need to set a password on haiku by running passwd from the terminal.
Doesn’t haiku also have nfsv4? Or is that client only?