The only assumption I need is that P_{f,g}([C,C+d]) >= h(d) > 0 for some arbitrary monotonic function h(d). This comes directly from the p-value formula.
I.e., for any d, there is a finite probability of finding an A or a B in [C,C+d]. I don't actually care what the shapes of f or g are at all beyond this - as long as this probability exists and is bounded below (in whatever class of functions f and g might be drawn from), it's all fine.