Hi, mocore!
Are you asking about boot option acting like copy2fs.flg? To set it manually during boot process from the boot menu?
I can agree with you, such an option will be sometimes usefull. But as i can see in "/etc/init.d/tc-install", "/usr/bin/tce-setup" and "/usr/bin/tce-load" there are no such option now.
You can modify scripts and make custom boot option, and then reassemble rootfs.gz. Working with rootfs.gz is wonderfully described in the wonderfull Corebook (really great one!). If I am not mistaken, in the TinyCore Wiki there were something about custom bootcodes.
Frankly speaking, such modification will be interesting for me too, so if you will have some results, share them please. Or maybe i will be able to help you.
Wish you success!