There are alias interfaces in linux, named things like eth0:2. To create them, you can just set an IP on that pseudo-interface and it will be created if it does not exist.
First thing to try is maybe this is supported in UCI by default (though somehow I might doubt it seeing as a multi-ip dynamic setup is probably rather uncommon these days). To try this, I suppose you may just create additional config interface sections, and set the "ifname" to be "myinterface:0" or "myinterface:1" etc. But for this to work, it would have to know something about the client ids.
In your case, you want the dhcp client to set them. It appears that it is possible, but may not be supported in every dhcp client, so you may have to install some more fully-featured dhcp packages if you need this.
See here:
https://dev.archer.edu.au/plone/Members … s-in-linux
To get the alias interface (eth0:0) up using dhcp I then ran the following command
ginga:~# dhcpcd-bin -I 01:00:1e:4f:ab:26:94 eth0:0
err, eth0:0: SO_SOBINDTODEVICE: No such device
ginga:~# dhcpcd.sh: interface eth0:0 has been configured with new IP=137.219.45.220
where the second argument is the HWADDR of eth0 incremented by 0x01 and prefixed with '01:' (as per the dhcpcd manpage - man dhcpcd-bin - search for clientid). You can see that I got an error, but the interface still comes up with the addy given, so I'm calling that a win 
In udhcp it seems that the clientid is set by the -c option. So you might be able to do this:
udhcpc -c 01:00:1e:4f:ab:26:94 -i eth0:0
Where the "94" is the last digit in your mac address incrememted by 1 for each new client ID you need. For subsequent ones, not sure if you keep incrementing both digits. But best get one alias working first.
You may need to install a more fully-featured client like dhcpcd, if udhcpc does not support adding the alias interfaces.
And once you get this working manually, you will need to make a startup script that runs the appropriate dhcp clients.
