GRUB is a multiboot loader that can run most OSes like Windows, LINUX and many more. You can now set up a section in the bootloader to run RDOS.
You can found information about GRUB here
A floppy based GRUB image and sample MENU.LST for RDOS is here
After you successfully installed GRUB onto your harddisk, just add the following lines in a new section in MENU.LST:
This assumes you have installed RDOS on c:\rdos and you want to run demo.