Thanks for the pointer to the boot.iso. I got Fedora 10 to boot to a network install, and only added 20MB to my usbstick.
Here's what I did -
Download and extract Fedora-10-i386-DVD.iso to your website (there wasn't an option in the boot to use FTP, only CD/DVD, NFS, and URL). I created a fedora10 directory in the root of the site.
Browse to images\pxeboot
Copy initrd.img and vmlinuz to UBCDROOT\fedora (create directory)
Add to menus\bootdsk2.cfg
MENU LABEL Fedora 10
I had to add the changes to my ubcd image, then "burn" it back to my usb stick.
Boot to UBCD, select DOS/Linux Boot Disks, then Linux Boot Disks, then Fedora 10. When prompted for the URL, enter http://sitename/fedora10
(or whatever dir you put your extracted files into).
I did have an issue with the video (USB LCD got scrambled) when it first tried to load the GUI part of the install, but when I tried again, it worked.
I ran this twice, with success both times, but if someone else wants to follow these steps and confirm the results, I would greatly appreciate it.