What the segment registers actually allowed was a primitive form of relocatable code: the 8086 didn't have PC-relative addressing modes, so instead what you got was 16-bit absolute addressing that was segment-relative. DOS could load .COM files (which had a 64KB limit and used the "tiny" model where all the segment registers point to the same 64KB region of memory) at any offset in conventional memory without needing to perform any runtime relocation. Combined with the lack of any header this made perfect sense for small utilities that needed to execute quickly.
I've never bothered to check this, but recently I was told that COM files could be >64K. The DOS loader would just keep loading data until the real 640K end of memory was reached. The program itself would be responsible to modify segment register to gain access to the unreachable parts of itself.
It seems technologically possible, as a program could always assume memory above its own segment (up to A000 of course) was free.
I can't speak for DOS support but I love building COM binaries on Linux which exceed 64k thanks to SFX which allows us to respect the GPL with source code bundling.
