As an entrepreneur who can honestly say that SEO, SEM, etc. is not my first rodeo, I can certainly understand where you are coming from. We want to know that everyone has "skin in the game." That in and of itself is not bad thinking. Certainly, you have had an employee who did not have skin in and you were wishing you had devised a better pay plan when they were not producing. I can tell you it has happened to me a few times.
In the context you mentioned of having someone after hours who works for you and is provided a sales incentive it begs a couple of questions: First, you are on SEOmoz and many people own companies but many are employees who work fulltime for companies. I would not want one of my employees taking a midnight gig for a potential "100K" payday due to it impacting their performance for which I am paying the 941 wages, bonuses, vacation, and health care. The reasons are obvious. Beyond the obvious, these employees learned at my company and we have ways of doing things that we believe to be proprietary. It becomes too tempting to borrow here or there (an example would be if they have access to multiple software programs we license and use them because they are used to using them). And, if I then found that they got the opportunity through SEOmoz, I think I might have to go for a really long run.....
In my opinion, you are better served finding a good firm and having them first assess the situation and let you know what they can or cannot do and the costs.Having worked in a situation where I ran a company on two occasions for a small base pay and percent of profits, I would not do that again. The main reason first: the argument is pay for performance - the reality is either they cannot afford you or they do not value you if you have any track record at all. In both cases the companies made more profit than ever before and, amazingly, there were reasons the profits would be less than promised.
If you search out several firms and interview them as if you were interviewing your successor, you should be able to find a good firm, with a good track record, and a diversified group of referrals that they have worked with over time. I know for a fact on SEO moz there are more than a few. Then, do as has been suggested and be very clear about what the milestones are so that you can monitor exactly what is happening and the results that are being achieved. Also be aware that sometimes as entrepreneurs our expectations can be overblown so be clear with the firm so they know what they are getting into.
If however you find that person who will moonlight for a share of the profit, remember that you can still lose and lose big. If you get 6 months down the road and realize they are never going to work out and you have wasted the time, it will be a year before you can retool. What will be the opportunity cost for that??? What would you have been willing to pay in retrospect to have done it otherwise?
I wish you luck.