Put something like the following in your RADIUS user table:
username Password = "UNIX"
User-Service-Type = Framed-User,
Framed-Protocol = PPP,
Framed-Address = ip.to.give.him,
Framed-Netmask = 255.255.255.255,
Framed-Routing = None,
Framed-Compression = Van-Jacobsen-TCP-IP,
Framed-Filter-Id = "std.ppp",
Framed-MTU = 1500
It should -not- be from the usual pool assigned to the portmaster.