There were loadlin and grub4dos back in the day. A more recent development is doslinux (https://github.com/lpsantil/doslinux https://github.com/lpsantil/doslin

There were loadlin and grub4dos back in the day. A more recent development is doslinux (https://github.com/lpsantil/doslinux https://github.com/lpsantil/doslinux) but it is far less complete. But more hackable if you just want to see something work.