As far as I know Microsoft lock their Operating Systems so that you cannot run them directly from an external storage device. Microsoft say something about it not being a 'primary means of storage.' More like them making an attempt at stopping people running multiple computers of on copy of the OS, as allowing them to install it on any storage would allow people to make loads of copies. Hence, Linux does not have this problem. I personally do not do much gaming, and have never needed Windows on my MacBook Pro, so have not dabbled in the whole 'Windows on my Mac' thing. If you really do have a problem with sticking Windows on the MBP's Hard drive and booting either natively or through virtualisation, then the following links may help, although the guides are designed for XP and I have no idea if they will work on 7.
Prepping the install CD (scroll down)
Installing
As I said, I cannot guarantee this will work for 7. I hope, however that I have helped in some way.
It sounds like the USB drive doesn't have the drivers to boot the other macs. :-(
The fastest path forward is to simply erase install a basic Snow Leopard OS onto the failed macs. While this is happening, do download the 10.6.8 Combo update to your USB drive just in case it's needed.
Don't bother running the updates unless the Lion installer forces you to get to a higher version than your 10.6 installer delivered.
Once you boot into Snow Leopard - you can try again to mount the USB and execute the Lion upgrade package. I don't know if it runs well from the USB or needs to be copied to the internal boot drive.
There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.
As an alternative - you could try instead to transfer the recovery partition, but this may not be universal (include the drivers the older macs) either.
There is a step-by-step recipe here for copying any bootable volume to one file on a USB drive.
If you are curious or feel it's worth a shot, image the recovery partition from your Lion mac.
You should be able to boot from DVD and use disk utility to make an equivalent partition on the "non bootable" mac and drop the recovery data to get a minimal bootable system and avoid a full Snow Leopard install.
Unless you are familiar with Disk Utility and the steps to capture, the reinstall option might be more likely to succeed on first attempt. I certainly don't know if this partition is customized by Lion and not universal so I've made it an aside for the curious.
Best Answer
It is not a question of whether this will work, but rather that this is not a wise idea. Basically, ordinary flash drives are not designed to withstand the amount of reads and writes that an operating system will require. Certain versions of linux provide a solution referred to a persistence. This is where the operation system creates a temporary drive in volatile RAM which can used be for read and writing. When you shutdown the operating system, the contents of this RAM drive is written to the flash drive. When you restart the operating system, the saved data is read from the flash drive into a new RAM drive.
There are many utilities that can create a flash drive version of linux with persistence. However, the few I have tried would not boot on a Mac.