Scuppers aren't good for aerators unless you're going to keep a (slow) sprinkler valve setup.

Personally with aerators, I like to have access to my fill pumps in case you have a priming issue and have to crack it open to get the pump primed.

Yes, the scupper would definitely fill while driving unless you have a sprinkler valve that stops the flow or if you have a vented loop. That's what I use for my bow sack since I got rid of the stupid sprinkler valves.

The best/cheapest vented loop is on ebay for about $5.
http://www.ebay.com/itm/1-PLASTIC-OU...ZUVPtV&vxp=mtr