Mat,
I have been thinking of the exact same thing. I have a 100 L rectangular bladder I use to quickly fill our tank but have been thinking of getting a slightly bigger one to install where you indicated. I think you could daisy chain the two tanks together so you don't need another fill or vent. Essentially the hose would run from the existing tank into the bladder and then a hose would run from the bladder to feed the pump.
My biggest concern is how to attach it.
I have also considered two other options. One is to loose the bottom two drawers an install about a 200 L hard tank(50 G). But I don't want to loose the storage. The other is to remove the way too big water heater and either just put a water tank there, probably around 150 L rectangular would fit. Or a smaller water heater and a 100 L tank.
Anyone else have any ideas? We plan to tackle some of these upgrades when we get to Puerto Rico or St. Thomas in a couple of months.
Fair winds,
Jesse