I think it's a great idea and if I had the room I would have done it ages ago.
I can, however, see Matt L's cautions about oxygen. If your big container full of water is a closed system with no method of adding oxygen in that area you may run into problems with that water losing oxygen due to bacteria and such living in that space.
People that use coils of tubing buried under ground to heat/cool sometimes have this problem, if the water is moving too slowly by the time the water gets to the the far end of the tube it's lost all it's oxygen and begins to stink of sulphur thanks to anerobic bacteria living in the low oxygen environment of the tube.
So you pump oxygenated water into the system and oxygen free water flows out. The efficientcy of your system in terms of it's ability to oxygenate your water will come into play. You may want to find out ahead of time if your system can handle the need to replace all of that oxygen.
As far as water changes, why should they get smaller (reletivly) or become a hassle? Put a couple of valves on your storage tank, when it's time to do a water change, close the valve sending water back to the reef, open another valve to empty the big tank, put in new water and salt, mix, and open the first valve again. Simple, easy, HUGE water change.
Good luck!
Jon