Regardless of the video content - you're asking people to put a commercial for your company at the beginning of their video, even though their video has nothing to do with your company. Then you're asking them to go out and promote the video, and by extension, promote your company. There is no compensation up front for either of these things, only the promise that there might be some later - that's the definition of spec work. Because it's done as a contest with a single winner the reality is that the majority of people will get no compensation at all, while your company benefits from every single exposure that all of their videos generate.
You say the aim is to get other companies to hire your company to set up a viral marketing campaign, but it seems to me like your company is not actually doing the viral marketing at all - you're looking for other video producers to do it for you. If I were a company looking to hire someone to do a viral video marketing campaign why would I go with your company instead of the person who actually produced and promoted a successful viral video?
Or, to look at it another way - what if I were a company looking to produce a viral video campaign, and I came to your production company and said "why don't you guys put our logo up front on your own videos, you go out there and promote them, and if you manage to get the most views compared to the other companies we're asking to do this we'll pay you a day's wages?" Would your company take the job?