What are you trying to solve? In my view the problem is that people don't understand how system works and waste resources by keeping them unused.
With your solution - no matter the default setting on mex-storages - when people will get they can "enable mex to be storage" they would just do it to feel happy they have lots of metal - same as now.
Why not make some annoying thing appear on screen (thinking about Microsoft Clippy) when you have metal unused? Bigger and bigger the more metal you have unused. This can be mildly annoying for advanced players that store to plop a desolator - but even that could be fixed by making a rank check before annoying too much...