The chainloading of older DOS (PC-DOS 5.0 - PC-DOS 7.1, MS-DOS 5.0 - 7.00 beta, Novell DOS 7.00, OpenDOS 7.01, DR-DOS 5.0 - 8.1) is added to Grub4DOS recently by me.
For me, it can be count as experiment and some sort of retrocomputing.
As an experiment, I want to test if DR-DOS 7.05 OEM can boot from FAT32 drive. And the result is that, although DR-DOS 7.05 is able to access FAT32 partitions, it doesn't able to boot from it. (Assume my implementation is correct enough to boot it)
As a sort of retrocomputing, you can boot older DOS on a FAT16 (non LBA) partition for fun.
PC-DOS 2000, 7.1, and MS-DOS 6.xx ibmbio.com/io.sys are automatically detected by chainloader, no switch is needed.
For others, it is needed to specify "--pcdos" or "--msdos" depends on the filename.
For example, chainloading MS-DOS 5.00:
chainloader --msdos /io.sys
chainloading DR DOS 6.0:
chainloader --pcdos /ibmbio.com
Have fun and a nice day.