Could we have a tool that could select triangles/linework to the left or right of a line, Then relayer or delete object outside selected line
Select by polygon if you select an open CAD line does something like this already - it creates a polygon by closing the open line and then looks to see what is inside or outside it and deletes what you pick - not obvious maybe but it does partially work depending on the polyline / polygon that you are working with
Left and right I guess is trickier because some of those lines may not be totally within the limits of the selected line - maybe partially is an option on how to do this
Give the above a try and see if it works for you
Alan