I love the OP's idea!
While I agree with your overall analysis and your proposed solution, the flaw in it is that EVERY slot in every window that involves inventory... is a database field. To add that many fields may not be as easy as you profess...
It's always important to keep in mind that unless you're working on a PARTICULAR product, it's never 100% possible to guess just how long a specific task would take.
That being said (and believe this or not), I was an MMO developer for several years before becoming a usability designer and trainer. Adding fields, columns, or rows into a database is perhaps the absolutely most simple thing that can be done in an MMO other than just changing information in already created fields. If you can add a row into Microsoft Excel, you can pretty much do the same thing for any database in an MMO.
But, this would also require new art, new UI elements, and QA testing, as everything in an MMO does. So, if they decided to put 5 Devs on this right now, there's still no way you would see this inside of a couple of months.