Now I left the initial line of code there and just added
tce=sdb1 waitusb=20
(I chose it deliberately long so I could observe the boot process better) and pressed ENTER. It does boot, but I couldn't notice any delays for waitusb. When I got into the desktop, it seemed to remember something, like my wi-fi settings, the bookmarks on my firefox etc. I can run applications that are already "installed" in my usb stick by running, for example,
$ tce-run firefox-ESR
but the Appbrowser that's set to
http://www.gtlib.gatech.edu/pub/tinycore won't let me browse for new applications or let my uninstall any of those I have already installed (in the usb-stick).