In my work of porting the MS-DOS v4 kernel to NASM I came across uses of eg "ROM SEGMENT AT 0F000h" [1]. 

In the manual, NASM's OMF -f obj output format is said [2] to have a SECTION ABSOLUTE= extension. Unfortunately, this doesn't do what I want it to do. It is described as follows: "The obj file format also allows segments to be declared as having a pre-defined absolute segment address, although no linkers are currently known to make sensible use of this feature; nevertheless, NASM allows you to declare a segment such as SEGMENT SCREEN ABSOLUTE=0xB800 if you need to. The ABSOLUTE and ALIGN keywords are mutually exclusive."

What I want is for "jmp far label" and "seg label" to be supported for absolute sections. To my understanding this support belongs into the assembler, as it knows the final segmented address of the absolute section.

Lacking this support I worked around it by explicitly specifying the absolute segment value to the jumps and I used the ABSOLUTE directive to place labels [3].

[1]: https://hg.pushbx.org/ecm/msdos4/file/28e097f848ba/src/BIOS/MSHARD.ASM#l59
[2]: https://www.nasm.us/xdoc/2.16.03/html/nasmdoc8.html#section-8.4.1
[3]: https://hg.pushbx.org/ecm/msdos4/rev/b35e275a8dae

Here are two test cases. The jmp gives an error and a panic. The SEG seems to be supported but does not relocate correctly: The assembler writes a zero and no relocation.

$ nasm -v
NASM version 2.16.02rc2 compiled on Oct 12 2023
$ cat test1.asm

        section CODE
jmp far abslabel

        section ABS absolute=0F000h
        resb 38
abslabel:
$ cat test2.asm

        section DATA
        dw foo, seg foo

        section ABS absolute=0F000h
        resb 42
foo:
$ nasm -f obj test1.asm -l /dev/stderr
     1
     2                                          section CODE
test1.asm:3: error: far-absolute relocations not supported by OBJ format
panic: test1.asm:3: unrecognised segment value in obj_write_fixup
$ nasm -f obj test2.asm -l /dev/stderr
     1
     2                                          section DATA
     3 00000000 [2A00]0000                      dw foo, seg foo
     4
     5                                          section ABS absolute=0F000h
     6 00000000 <res 2Ah>                       resb 42
     7                                  foo:
$