Is there a front derailer type that can be mounted on a frame without braze on or fitting on the seat tube, or clamping?
'E type' is a front derailleur attachment standard. Some frames provide an E type mounting point directly, some require a 'back plate' that provides the mount point that is clamped by the bb cup.

As far as I can tell, some back plates are clamped by the cup and have no other provision to prevent rotation. There is not very much fore or aft force on the derailleur so presumably this is not a problem. Other back plates have an additional hole that matches a threaded hole on the frame, so that an anti-rotation bolt can be fitted.