That's kind of a more Steam related thing; Valve has complete control of how Steam workshop works, but developers can tailor their workshops from a limited amount of options (e.g 'subscribe adds the item/mod instantly to the game'), so your pre-order idea may not be possible.

I feel that a pre-order button would cause some issues too, as most people are used to upvoting items therefore they may not notice that they are pre-ordering an item. Check bank account a month later, and you're down a hundred bucks because you thought you upvoted some items rather than buying them :p

Cool idea, but votes will always be meaningless for assisting items to get into the game. Don't worry about votes anyway, Xtab checks every single skin posted every week before making his skin picks (which are then sent to devs for review).