My conclusion is that TC can not support booting from different partitions / OSes, unless by manually building grub.cfg.
Hi, clauder!
I think You are right, and it is by design. Look at the project home page:
About Our Project
Our goal is the creation of a nomadic ultra small graphical desktop operating system capable of booting from cdrom, pendrive, or frugally from a hard drive.
Robert Shingledecker, December 01, 2008
Bootlader, kernel and system all can be placed on separate physical drives, including network ones. Every os-prober is built using some assumptions, but TinyCore don't intends to put any limitations upon Your desired system architecture, remaining minimalistic and top flexible.
I can mention, that system configurating script (tc-config) apply certain attempts to resolve undefined tce location according to the environment, present at boot time.
Have a nice Core!