Blending sides of cubemap when mipmapping

Is it possible for one side of a cubemap to influence another side when generating mipmap levels? This would come in handy for using a cubemap as a diffuse lighting map by using a biased mipmap level texture lookup in a fragment program. If you try this without letting the sides influence one another, you get hard lines along the corners of the cubemap. Perhaps there is an extension that does this that I have overlooked…