I think that shoehorning a contest entry form into the existing Q&A format will create a bad precedent, particularly in the case of new users. And the idea of a sponsored question sounds like it could create a conflict of interest.
Why easy questions are bad for the site
Questions that are easy to answer by anyone are, by definition, bad questions because Stack Exchange is designed to produce good answers. Unless I'm missing the point here, it sounds like you want to ask some sponsored questions that will be easy to answer.
Attracting new users to the site means bringing volumes of them here. The Stack Exchange system is set up to actively discourage users who don't want to work in the Q&A format, leaving only the ones who will generate good content. These two obviously work at cross purposes, and I don't envy anyone the task of getting new users here.
On Stack Exchange, good information will be easy to find; as the good answers are upvoted, they'll rise to the top of the page. Writing good questions and answers is supposed to be a difficult thing, not an easy one. New users - and getting them here is the point of this exercise - will see a question like this and assume it's representative of the site. Allowing questions that are easy to answer because they're sponsored would be watering down the site.
The cycle accessory question you link to would almost certainly be closed were it asked today. It's a chatty question that serves no real purpose. What do the upvotes mean on those answers? That someone liked these accessories? That the upvoters agree with the answers?
Similarly, introducing a Q&A thread that's easy to participate in would defeat the purpose of the site. We want questions that produce answers by experts. If there are many answers possible, then, a few glossary community wiki questions to the side, such a question isn't a very good one or it isn't being asked properly.
Sponsored questions and conflicts of interest
Let's say that a way can be found to get past all of that. Well and good. If, say, Tektro wanted to sponsor a question about brake levers, would mods be bound to keep it open? What about users who feel that they make horrible components? (Personally, I love my Tektro brakes, but I digress.)
Would users be bound to avoid sharing their extensive knowledge if it's bad for the sponsor? Are we stuck with a sponsored question if it's a bad question?
Similar to how blog functionality was introduced with Blog Overflow, and moderator elections are carried out on a dedicated election page, perhaps a new functionality for contests can be created. This not only wouldn't compromise the site but it would allow for greater flexibility in building the perfect promotional/contest engine.
While most efforts to find new ways to use the Q&A format seem to end fairly badly, some do not. Witness the glossary (which originated, as far as I know, on Cooking) and the citation-heavy format on Skeptics. Perhaps I'm missing the point here. What exactly are you hoping to achieve here? What organizations are interested in participating? If we know more, we can make better suggestions.