Any chance of implementing 99.999 resource units cap? I rather liked it in HOI3 demo, and can see on the forums that it is also implemented in full game. Would like to see that feature in AOD also 
I'd like to see this kind of stockpile management system implemented; via techs, it's a good ideaBut it might require resource outputs to be increased slightly to compensate (via techs or otherwise).
A different type of idea: instead of a cap, make the stock pile decay, like a 0.1% daily or whatever. If you don't use it, or you don't trade it away, it will slowly vanish...
A variant of this idea is to charge a "maintenance expense" in money for example, so that the bigger your stock, the more you must keep paying daily, simulating your storage and maintance effort.