This is not possible, currently.
There are definitely positives and negatives of having an internal pool. If you have separate RAP controllers, you can make every pool the same, because the ip address of the RAP is normally only significant within the controller. 99% of RAP information is obtained through the controller and NOT directly from the ip address of the RAP, so that you don't normally need a routable ip address.
The reason why you WOULD need a routable address is if you wanted to forward packet captures on a RAP to a management station. The IP address of the RAP would need to be routable in that scenario. All you would have to do is to have an ip interface on your controller and have the pool give out ip addresses on that subnet.