First I tried to build our code in Cygwin using the Windows version of libusb. I found that fraught with complexities of all sorts and eventually decided the Live CD approach sounded a lot easier. Besides, we wanted the Live CD/bootable USB stick anyway for some later plans.
Theoretically there are small Linux distributions (the most famous being Damn Small Linux) that can be used for this sort of thing. As soon as I started getting into that, I found that DSL is no longer maintained, the documentation for it is insufficient and the pieces that do exist contradict one another. I struggled to resolve dependency and version issues in porting our code to DSL and finally gave up. By that time, I had already discovered how to make an Ubuntu Live CD, and so I delivered one with the first piece of our code to our partner.
But I really wanted a much shorter boot time. I don't need X Windows or networking or OpenOffice or a web browser. I'd prefer to have a development environment on there in case the code required modification but even that is unnecessary.
In the past several days I've tinkered with about a dozen Linux distributions claiming to be "small" and found them all deficient in one way or another. I've tried dozens and dozens of permutations of dumb little tricks involving VirtualBox and QEMU and Ubuntu Customization Kit and burning CD-Rs and USB sticks. I've looked at what feels like hundreds of different web pages and blog postings, each claiming to have an authoritative and trustworthy solution to my problem. Each involves failures to account for discrepancies between versions, or the document I've found is old and inapplicable to what I'm doing, or the author made several minor assumptions that don't work in my environment.
Currently I'm looking at something called Ununtu Mini Remix which looks promising. It's looking very good so far, as I am remastering it with the information on the Ubuntu help website. Adding a shell script to /bin to make sure I can, and adding an "echo HELLO" to /etc/skel/.bashrc to make sure it appears when the disk boots into a bash session.
Everything was going great and then mksquashfs got hung up on the proc directory -- AH, this happened because when you finish a chroot session you must do three umounts (dev/pts, proc, sys) even if you weren't aware of having mounted them. Apparently chroot mounts them without telling you. Umount those in the chroot environment, exit, then umount edit/dev, and the mksquashfs goes just dandy.
So my two dumb tricks in /bin and /etc/skel/.bashrc worked like a champ in VirtualBox and now I'm going to try to make a bootable USB stick. Ubuntu's Startup Disk Creator likes the file (it's very picky about what ISO files are considered bootable) and the USB stick works great in my Windows laptop. Now we make the Angry Birds WHEEEE noise, however dumb some people might find the game. The next step is to make my tweaks into a Deb file using this HOWTO so they go in painlessly.
Grazie mille to Fabrizio Balliano for creating Ubuntu Mini Remix.