Is there a front derailer type that can be mounted on a frame without braze on or fitting on the seat tube, or clamping? As the question says, is there a front derailer type that can be mounted without having a fitting or braze on on the seat tube, or without clamping to the seat tube (because it's a weird shape, or made from a material that one cannot safely clamp too)? To my knowledge there are clamp on derailer that expect a seat tube that is circular in cross section and braze on derailers that expect a brazed on fitting on the seat tube. Finally there are derailers that are clamped on by the right hand bottom bracket cup/ring, but as far as I can tell they also require an additional fastening method on the seat tube to prevent them from rotating around the bottom bracket. I think these are sometimes called E-Type or Bottom Bracket derailers.

'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.