Actually I've implemented four kinds of border extensions, vil_border_constant, vil_border_geodesic, vil_border_reflect (symmetry w.r.t. the closest edge, I guess this is what you are talking about) and vil_border_periodic (the right edge is linked to the left one and the bottom one to the top one, like a "double" torus). I could not think of other very common ones.
I like the idea very much! It's a nice, modular design.
I would say, go ahead with it in CVS vxl.
(I'm thinking of yet another typical border extension method:
use reflected values, i.e., the border acts like a mirror.)