Ball pythons are nocturnal animals so 'basking' isn't really a big thing for them. During those prime basking hours a bp is curled up in a dark,cozy little spot waiting for the sun to go down. As long as you keep the temperatures right and have a hide on each side, that should provide them with the ability to thermoregulate themselves. If you do put the rock in there make sure to get a temp gun to make sure the rock isn't retaining high temperatures which might burn them.