@User: I don't know how the PDN scripts are written, but as far as I am concerned your design should be able to use any one to all four of the provided supplies. The power ring itself is fixed, and not part of the user area per se, because it has to match the connections being made to it from the chip top level. But it sounds like your issue is just to keep the power stripes in the interior limited to only vccd1 and vssd1, which ought to be doable. You will need to ask Mohamed Shalan's group. . . Try Marwan Abbas (if not him, then at least he can find you the right person to talk to).
Otherwise you do have a couple of options: (1) Merge together vccd1 and vccd2 (assuming that's possible with the PDN scripts---again, it ought to be possible to do that, but doesn't necessarily mean that it is); (2) Use the caravan chip, in which case you can design the power ring any way you want, the only issues being that you will have to manually connect up the power supplies to the wrapper edge, and you will lose 11 GPIO pins. But I think it's better to just raise the issue and flag it as a major error if the PDN scripts cannot accommodate what you want to do.