Ralph- I've been researching fuel cells and aux gas tanks for a few months now. I've learned some interesting stuff but I won't have any practical knowledge until I install my own on the GS.
231 cubic inches is one gallon. That's a cube about 6"x6"x6.5"- not very big when you think about it.
Your unsafe, semi-clever idea isn't that practical. When you gassed up, your fuel would have to flow freely into the aux tank from the main tank. That's hard to do with fuel lines being the diameter they are. It takes most non-fuel pump cells an hour or more to fully drain 4 gallons into the main tank with normal fuel lines. You can get a bigger diameter fuel hose, but I'm not sure if it's meant to hold gas or not. I think this would also create trouble with venting considering that there will be expansion and gasses that want out that may create pressure on your gas line to the main tank, i.e. no fuel to the aux which goes to the carbs unless you make a vent hose. It would be easier than installing a new fuel cap.
Usually you keep a cell shut off until you're down to reserve when you're filling the main tank via gravity. The theory is that you don't want to overflow your main tank through its vent. The cell should have a vent hose too. I'm personally torn on how to connect mine. A commercially produced aux tank for my bike connects at the bottom of my plastic gas tank through a hole you drill in it. Holes in gas tanks make me nervous but with the fuel pump, you definately have to get the gas into the tank first.
Don't know if it's such a good plan to try to install a cell under the tail- there's not that much room, especially if you're putting luggage and stuff on the back, weighing it down. Are you looking for a permanent addition here? It may not be that easy. BUT I read about a guy who had a metal shop fab a cell for his goldwing out of aluminum. The key was welding baffles into it so the weight didn't shift around so quickly. His price was ~$250. You might be able to get someone to do that for you per your underseat specifications. Definately make sure that you can fit enough volume in there to make it worthwhile.
There's a cell for $50 at http://www.goracin.com/ in the clearance section. It's not pretty but it might give you an idea. Otherwise Summit Racing has a good selection of cells, fuel caps and stuff. http://store.summitracing.com/
My vote is still for a tacky box fuel cell on the passenger seat, mounted with quality straps or attached via a tail rack. Then you could just splice into your fuel line and install a valve. Then you could just go off of mileage to figure out when to switch. It's a hard call though since there's not so much room there.
Edited because I changed my mind about everything I had just posted...
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Originally Posted by Dyscofool
I'll get a tank bag soon enough...
HA HA HA.