One time, we selected our vehicle, carefully loaded all of our luggage & installed car seats, and then found that there was no key! Someone must have taken it to "reserve" it, but not locked it!

It was very annoying to have to move everything over to another vehicle.
So, if you do decide to do this, please be sure to lock the door to save someone the hassle we went through.
Honestly, I don't think it's worth it, because the inventory is constantly changing, and the odds are just as great that after you select what you want and take the key, when you come back after getting your luggage, there would be something just as good or even better, and you'd want to switch.
Really, the key with Emerald Aisle is to be absolutely sure that you're fine with a mid-size vehicle, and if you find an upgrade to something else on the aisle, just consider it a bonus!