DHCP and dynamic ip allocation from a pool

Does anybody already implemented dynamic allocation of ips from pool? Or any ideas? In fact, main problem in native rlm_sql_ippool is that "freeing" of ip is done via accounting section - and there is no analog of Stop packet in dhcp (but it can be simulated, using Lease-Time) -- With best regards, Evgeniy Kozhuhovskiy, Leader of Services team, Minsk State Phony Network, RUE Beltelecom.