Was meaning to post exactly the same question! Enough evidence has been accumulated to convince me it can happen. The sad thing is that well-prepared natural childbirth is so rare, there might not be a lot of people prepared to answer OP's question.
Several birth stories in Ina May Gaskin's Guide to Childbirth detail orgasms or pleasurable sensations close to it during contractions or pushing - natural childbirth only, obviously. I doubt an epi could numb pain but still allow an orgasm.
Ina May devotes a chapter of the same book to the "Pain/Pleasure Riddle": she describes pleasure as possible when there is an environment in which the birthing mother feels totally at ease and private, in which she can kiss her partner, touch herself to encourage relaxation and ward off pain, and envision birth as the culmination of the sexual act (when you think about it, it really is).
In order to start thinking in that direction, it is necessary to eliminate from your mind all the horrific birth scenes and myths perpetuated by the media and by the systematic over-use of interventions in hospital settings. Birth nowadays is seen as something inherently dangerous that must be technicalized, not the natural, wondrous and mostly safe biological miracle is actually is.
I had an emergency, pitocin-induced but otherwise unmedicated birth for DC1 - and it was horrible. My goal for DC2 is to have a prepared, natural childbirth - the huge difference will be in my mental state - I know it will be wonderful, and I hope there will be an orgasm!
