My own second answer:
1) Although I applied what was suggested at the end of the thread mentioned before,
Why isn't persistence working on Lubuntu 12.04 Live-USB? ,
my first lubuntu installation is still not persistent.
2) I used the existing USB installation as a springboard to install lubuntu on another (larger) USB stick. No problems with the installation process, but the new installation is also not persistent. And even more interesting, I cannot find either of the following files or directories (as before, using the command "sudo find / -name [...]":
syslinux, isolinux, txt.cfg, text.cfg
(someone less sanguine than myself might perhaps insert some swear words here)
Note: as with the first installation, there is a directory "cdrom" in the root - but with the second installation it is shown as empty.
Back to the original question...