Yast and GRUB are two different things, Yast can find other systems but it cannot boot them. On the other hand, broken GRUB can't find systems and cannot offer to boot them. Additionally, the install media (Linuxrc) offer to boot from disk (installed systems), but it only tries to load the installed bootloader (GRUB in some cases). So, I'm afraid this is not possible with the tools we have now.