Force a read-only mount of startup disk
The install CDs took a lot of shortcuts to deal with read-only boot volumes. One of the tricks is to mount the swap partition in a ramdisk - yes, it sounds rather pointless to swap your RAM to another part of memory, but I expect it was easier to do that than to rewrite the kernel to not swap at all. Make the GUI single-application and you won't need swap anyway.

If you are making a maintenance drive rather than a one-task USB installer I would recommend a spinning external. Faster than most USB thumb drives and it will have enough space to recover data. Get a 500Gb external, partition it into a 50Gb boot volume with CCC, DiskWarrior, DataRescue and whatever else you like, a standard recovery partition, and the rest for user data.