I have managed to achieve what I wanted by adding the following to the MATLAB toolbox file RSETTileManager.m (see attached file) Could this could be incorporated as a standard feature of imtool?
%% Allow line features to be retained other_objs = findall(obj.ax,'type','line'); for i = 1:numel(other_objs) uistack(other_objs(i),'top'); end %%