You might be thinking of UMSDOS. It was pretty neat; it allowed for instance C:\LINUX on DOS to become the root filesystem when booted into Linux (which also booted from DOS through LOADLIN), with the DOS root directory (C:\) becoming something else within Linux (I don't recall the exact path, it probably was something like /dos or /umsdos). That allowed you to install Linux like any other normal DOS program (yes, it was normal under DOS for a program to completely bypass the operating system), without having to re-partition/reformat or even change the bootloader. It's the main reason that I ended up using Linux and not one of the BSDs; I could try out Linux without any permanent changes to the current MS-DOS install (and some time later, I noticed that I was nearly all the time running Linux instead of DOS, so I re-partitioned to give Linux a whole ext2 partition).

Yep that was it.