I used/sold nitrous recreationally in highschool (yeah, I know, but whatever I was a bad kid) and I don't mean whip-hits either - I bought it on eBay. So, having a bit of experience with it - I don't see it helping a lot in labor. It makes you really high and definitely makes you forget pain (but doesn't dull it really) but you often experience periods of extreme lethargy - I would be concerned that it would have more of a tranquilizing effect than an epi and slow down labor more. The best thing about birth was the experience of it - I remember every detail and it was amazing (I had an epi 20 hours in and was able to laugh with my husband and really experience the whole thing for that last hour) and nitrous would just dull it. IMHO.
~ Posted from my iPhone. Please excuse any weird typos or foul language the autocorrect likes to sabotage me. <3