A Couple of Thoughts...
Ric, I used to have the gas tank in the cockpit, by the transom, in a box constructed there. It worked well. I suppose venting the bilge to the cockpit could be OK. Conventionally, vents are installed high up either off the transom or very near the transom, and I suppose because the air there is clean and open. But as I said I had no problem when the gas tank was in the cockpit. Its an exposed location and there is a scupper drain. Other than your feet and legs bumping against the fitting make sure you mount the vent high up on the sidewall of the cockpit, as it could be an opening to the bilge if the cockpit fills up with water. Thats happened to me twice when caught out in a severe rainstorm. The floor scupper couldn't take the rain away fast enough and started to back up in the cockpit. One reason why bluewater boats have large and multiple scuppers and smallish cockpits.As far as the bilge water, if your bilges are clean I suppose it would be OK. My bilges are fouled and I would not want that water rolling across my cockpit floor.