I also haven't figured out how to only update firmware for specific devices. With the inventory_firmware and update_firmware commands, it looks for updates and applies all updates at once.
inventory_firmware_gui lets you select. I'm not aware of how to achieve this with command line only.
As for creating a local mirror, a simple rsync grabs it (I exclude everything I don't need)
Yes, I have something pretty similar :)
I'll attach the yum repo file I use for my local repo. A 'yum install srvadmin-all dell_ft_install' will grab what you need for firmware tools and OpenManage, and then a 'yum install $(bootstrap_firmware)' will grab all your hardware specific firmware.
Thanks, I'll compare it with my own to see what's wrong.