I think you should let on... first of all, it's honest (I know if I were the founder I would really want to keep an honest problem-solver happy and well-paid) and second of all what goes around comes around.
I don't know that it is exactly good karma to not let on, but it would be something to consider.
Would you really give the honest problem-solver what they deserve though? The last project I did at a large company that saved the company a lot of time and money netted me a 10% raise. Probably less than 1% what I saved them per year.
It's an ethical dilemma, but in my opinion the fundamental right to earn a living (basic economic freedom) trumps the letter of an overly-restrictive contract, _if_ the employee's outside activities don't impact the employer in a material way. That's not law, that's just my opinion, and I realize it could be a bit of a slippery slope. However I just don't see how a company can own an employee's time away from work (again unless the employee is competing in the employer's direct line of business). See the work of Brandeis.