I was able to moonlight as a senior, but it involved diabetic retinopathy screenings on a Saturday. I did not do any moonlighting in the traditional sense (in the ER or as a hospitalist). Some places let you moonlight by doing refractions or other similar tasks. I got paid really well with diabetic retinopathy screenings - it worked out to roughly 200-250 an hour, for only a morning's worth of work. However, our situation in residency was really unique.
I really would not go the route of normal moonlighting. In addition to what the posters have said above, you may be in major trouble if something happens that leads to a really bad outcome. Your residency program may not be able to support you or have your back, and worst case scenario you may find yourself in probation or even expulsion.