The rules of payment and overtime constraints are best asked to a lawyer - it's going to be dependant on your location and the nature of the work. In the US - it's not just the country, but also state regulations that can be variable, and unions also play into the mix in some scenarios.
The other question, from a more managerial perspective, that you may want to answer is - how will after hours work affect the salaried work that you are already paying for? Most salaried work is salaried because there is a reasonable expectation that the work is somewhat fluid and not conducive to simply clocking in and clocking out - most salaried positions have the expectation that workers may be asked to work some unpaid extra hours that are within their job descriptions.
If you pay them extra for doing work that isn't in their job descriptions - will there be a conflict when you ask employees to work the regularly expected overtime that is part of their salaried work. Theoretically, I'd expect you don't want to change the nature or the quality of the salaried work by offering this extra incentive work... so you won't be happy to hear "I can't stay late and do that... I have to finish my peice work." -- in particular because chances are good that you need both things to get done - both the salaried work and piece work/hourly work is important. If you have one person doing both then you suffer double the calamity if the person has a work/life issue can can't get something done or if some job requires a little extra attention.
It's worth pursuing the legal questions, but regardless of whether it's OK under the law, consider how you want to administrate this, and whether it's worth the added risk.