This is John Davenport at SpectoLabs. We have used AspectJ to create new seams (in Daniel's terms) in Java monoliths that can then be intercepted via http.

It all depends on what sort of bind you find yourself in. Having seen both the Hoverfly approach is far preferable, but one approach may be to use AspectJ, create the seam via http and then intercept using Hoverfly. YMMV.