In order to make it easier, perhaps you make first a USB install? (not an ISO creation on a CD).
Your target: "Need live-cd with USB-CD boot & USB-modem support. Target image size<30MBytes"
So my understanding is: you have a CD reader connected to an USB and another USB port for connecting a USB-modem.
This is what I have on my samsung nc10.
You could do:
- install TinyCore (from another PC) on a USB memory stick
- put that stick on your PC
- boot your PC from it; make all setups by using a LAN connection
- post here the result of "lsusb" when your modem is in your PC
- then make the usb_modeswitch install. See here
http://forum.tinycorelinux.net/index.php/topic,11509.msg61207.html#msg61207I know, this is not your target, but perhaps an easier target in my point of view.