One MAC, two IP with DHCP (dhclient) how?

[Peter B]

In an enviroment where the ethernet network interface is connected to a public
ethernet network (internet). And a IPv4 address assignment scheme is static.
One can simple add primary IP by:
  ifconfig <interface> inet 1.2.3.4

And an additional aliases with:
  ifconfig <interface> inet 1.2.3.5 alias

This alias IP can then be used within a jail(8) enviroment.       
So far all is perfectly fine.

But in an enviroment where the primary IP is retrieved or set with DHCP by
using:
  dhclient <interface>

How does one request an additional IP beside the primary one ..?
Setting an alias directly with ifconfig won't work because the upstream network
won't route it.

Should one set an explicit xid or Client ID somehow? and how is that done in
such case?
(option dhcp-client-identifier data-string; in dhclient.conf)