Both the assembler and the linker support ELF, for object files, libraries, and executables. The ELF format has been extended with a machine type of 6502 (naturally) to permit storing 65xx code in ELF files.
You can use any of your favorite existing ELF tools to inspect or understand files generated by llvm-mos. The llvm-readobj, llvm-objdump, llvm-objcopy, llvm-strip, and likely the other command line tools as well, work as expected.
Although they don't necessarily know anything about the MOS processor line, this also means that generic tools that process ELF files, such as this online ELF viewer, can read and dump information about MOS executables, such as symbol tables, fixups, and relocation entries.
There exists a more precise ELF specification for llvm-mos.