Do you mean ~something~ like in the attachment? If so, and especially with meshes, it may be easier to create a jpg of the same area and apply it as a material.
I know what you mean mate. You should still be able to do it with a material. I once created a material for a very odd shaped site and it seemed to work as expected. It just takes forethought when mapping the material. See the attachment.
Otherwise I fear you will have to use your favorite hole method and spend a lot of time readjusting all those funky mesh points.
Just duplicate the mesh, trace the shape with a fill, offset the fill 50mm or what ever you want your border width, then select the mesh, click on a node, select the "subtract from polygon" button in the pet palette and magic wand the fill. done