Your ideas are really interesting ones. Unfortunately, I can’t help much, because I don’t have much knowledge in IN/VAS. However, I’m pretty sure it’s possible to implement both ideas…your IN/VAS department should help you, as both scenarios fall in their domain.
I’ll also like to mention that one network in my country currently has a promotion which is almost like idea no. 2, whereby prepaid subs can exceed the value of their credit, but the balance is deducted at the next recharge.