Not exactly what you were looking for but an alternative.
Both the vpngroup address-pool command and the ip local pool command enable you to specify a pool of local addresses to be used for assigning dynamic ip addresses to remote VPN clients.
eg:
ip local pool test 192.168.1.1-192.168.1.25
vpngroup vpn3000 address-pool test
vpngroup vpn3000 dns-server 10.2.2.2
vpngroup vpn3000 wins-server 10.2.2.2
vpngroup vpn3000 default-domain cisco.com
vpngroup vpn3000 idle-time 1800
vpngroup vpn3000 password ********
Steve