freeradius address assignment from ippool

I'm trying to figure out how to assign IP addresses from an ippool to mobile units. Ideally, I would like a unit to receive the same IP at each request. There is no data source to use to populate static entries for each unit, so I would prefer each unit to get an IP assigned from a pool first time it connects and then keep that address.

Currently, I have the following pool:

ippool dapool {
    range-start = X.X.0.5
    range-stop = X.X.0.254
    netmask = 255.255.255.0
    # should have room for all IPs:
    cache-size = 260
    session-db = ${db_dir}/db.ippool
    ip-index = ${db_dir}/db.ipindex
    override = yes
    maximum-timeout = 0
    key = "%{Calling-Station-Id}"
}

I thought that choosing a key that is unique for the requesting unit would do the trick, but apparently not.

1) Is it possible to achieve what I want?

2) Is there any good documentation that I should have read instead of pestering serverfault? I can't seem to find much useful documentation on FreeRadius beyond what is listed in the config files.