When my /24 ran out I started doing this. I just added the new private network(s) to OSPF and the routes propagated through. The privates are routed all the way back to the core. Then I just added a src-nat rule to the core router. On each tower router, I just added another IP pool so when the publics run out on a particular tower, they get a priavte IP instead. Good temporary fix till I get more IP's.
add action=src-nat chain=srcnat comment="NAT Customers with Non Public addresses" disabled=no out-interface="Public - ether05" src-address=10.10.0.0/21 to-addresses="Public IP of your choice"
By the way, nice tower site (in the other thread) :)