Starting surface needs at a minimum a PreserveEdges options like Patch has:
Preserve edges
Clamps the edges of the starting surface in place. This is useful if you are using a curve or points for deforming an existing surface, and you do not want the edges of the starting surface to move.
(Added) This option in Patch preserves the untrimmed edges of the starting surface.
The ability to set the level of preservation individually for each untrimmed edge including “not preserved/free” would be excellent.