I don't have a sure answer for you and i doubt anyone does.
There is a crazy controversy about helmets in WATER crashes ALONE which you may know about or at least ran into in your searches. But i am sure your wearing one for rails specifically?
anyway, i would guess that the skate helmet would be fine. I don't know a whole lot about helmets, but a lot of their design is based on the speeds engaged in and the surfaces that are expected to be hit. considering this notion, wake helmets are certified for kayaking which is based on speeds slower than what we wakeboard and based on hitting your head on rocks in rivers. mostly i am telling you this for an f.y.i. kinda thing... not at all meant to discourage use of any type of helmet
If you plan on just trying out rails and dont plan to hit them often AND your helmet seems to be made of materials that are not going to soak up water, i'd GUESS that the skate helmet will be acceptable, otherwise, invest in a watersports helmet
i finally jumped and got me a wake helmet. i got a discounted one from Shred Ready. I got the PHLY (i have not used it yet) version http://www.shopatron.com/product/product_id=SHDPH/343.0.10087.10085.0.0.0
i believe the phly was discontinued because it is kind of a multi-sport helmet, for snow and water. shred ready seems to do mainly/only watersports helmets so i guess no one wanted to get multisport when they were looking for water only/specifically anyway... But i think the PHLY looks pretty sweet, for a helmet!