The game will go gold around mid-may (meaning the product will begin being made, DVD/BluRay/Statue/Artbook/Case/Instruction Booklet(if they have one)/etc) At this time no more items can be added to the product as the assembly line have been programmed/instructed to mass create this item.

So basically there are 1.5 months for changing things that go into the package. Now something like adding a pin or sticker decal to the box is simple if SE is the only person manufacturing the items (which they tend to do because they do have a whole "toy/figure" line-up of merchandise) however when you add a code or item that belongs to another company you have to start negotiations. These negotiations are usually about pricing, percentage cuts, creation costs, and net profit margins. They are not something that can really happen in that short of a time frame sadly. So I doubt, petition or not, this will happen...however! If you are truly dedicated to using Steam a better suggestion/petition would to be make Steam Codes available in future Collector's Edition expansions post-Heavensward. A petition like that is wholeheartedly possible if there is enough support behind it.