Johhny....I like your thought process, totally makes sense how it would work, albeit slowly. My issue is I've got a T22 so therefore so chillax seat in the walk-thru or even a seat there. My access the bag below the seats is under the center bow cushion, which would be completely covered by the bag on top of the seats and no way to get the lines down to the bag under the seat.
I also don't think I would need TWO bags worth of weight in the bow, I feel like that would ride really low in the front and be overkill.
How much lead do you have up in the bow? Any lead elsewhere in the boat? The always there thing concerns me as well.
My 650 SL bow sac has 3 ports and a vent w/ the push down spring thing. 1 port on top, currently my vent and 1 port on each rear corner/side of the bag, currently my fill is one and the drain is the other. I think I can make it work how I'm thinking without my work on the boat itself, just obtaining some more ballast line and a few more connectors.
Basically I'm thinking about having THIS stuff in both sides of my bow panels, where the cupholders are, with a valve in-line to open when I have it hooked up. That way I either use an additional bag on top of the seats or just pull my current one out from under the seats. Having a 2nd bag would be the easiest I agree.
http://carphotos.cardomain.com/images/0015/28/85/15908258_large.jpg