I missed this when it was initially announced. The source code for PC-MOS/386 version 5.01 is now available on github under the GPLv3 license. It requires the user to supply Borland C++ 3.1 in order to build, but there are binaries checked in as well, including a bootable floppy image.
PC-MOS is a multi-tasking/multi-user DOS clone. It was one of the first commercial products which used the 386’s virtual-8086 mode when it was released in early 1987 (but not the first, that was almost certainly CEMM in 1986).
It is worth noting that PC-MOS also supported 286 systems. It required custom hardware and also used the well-undocumented LOADALL instruction (see MOXMEM.INC).
The source code is unfortunately not organized in a very clear fashion; there are for example three separate and not quite identical copies of the PC-MOS kernel.
There is an interesting reference to “Bimini 2-SX” aka IBM 486SLC, which apparently had a bug affecting PC-MOS/386: “When executing a CALL or JMP instruction with a negative
displacement within a 32-bit code segment and an operand prefix override is used (i.e., to limit the target address to 16 bits), the resulting target address is computed incorrectly.” (see _386.ASM in the mos5src directory for additional detail).