Out of all the operating systems for the 80286 processor I found, only two make use of the protected mode's ability to load more than one segment for text and one for data. These are MS-DOS (through various DOS extenders) and Windows. All other operating systems I checked would only give one text segment and one data segment to each process and call it a day. Were there any other operating systems that made full use of protected mode segmentation on the 80286?
OS/2 supported “huge memory” on 286s. The New Executable format used for 16-bit OS/2 executables (and 16-bit Windows executables) supports multiple segments. At runtime, using the
DosAllocHuge function, programs could allocate more than 64KiB of memory at a time, and would get a sequence of segment selectors which could be used to easily access all the allocated memory. The process is detailed in section 9.2.2 of Gordon Letwin’s Inside OS/2.
Xenix 286 also supported multiple text and data segments; processes with multiple segments were called “large model processes” (the same terminology as was used with C compilers under DOS). See Overview of the Xenix 286 Operating System.
FlexOS 286 (and perhaps Concurrent DOS 286) also allowed programs to allocate multiple segments.
malloc could only allocate up to 64KiB at once, but programs could call it multiple times to allocate more than 64KiB in total in multiple segments.
I suspected Coherent 3 might have supported multiple segments, but it turns out that’s not the case, at least according to the Coherent 3.2 FAQ (question 7).