Wow man, you got a good head for ideas. I love you for that.
This can be done without trouble, at least in my eyes. Of course, I suck at Ruby, but the concepts I can think of...
Eat a certain item. This item will trigger a Common Event. The Common Event will trigger a Call Script to activate the EXP Double Script. It also activates a Timer. Once that timer expires, the common event will trigger another Call Script to disable the EXP Double Script.
Doubling the EXP shouldn't be hard at all. You just need to find the variable in the scripts there (Which you'll bound to find if you search in the Battle areas or something), and then make an algorithm (or formula) doubling the value ( x*2 :x ).
That's what I think anyway.