I've got something working - quite hacky, but anyhow:
https://github.com/bitRAKE/fasm_playground/tree/reloc

Less than 100 lines of code:
https://github.com/bitRAKE/fasm_playground/blob/reloc/SOURCE/__reloc.inc

It maps a virtual address space to basically anywhere in memory - bypassing all checking. Since we want the relocation information, it's mapped to there. To say this implementation is fragile would be an understatement.

I know it's not what you ask for, but you could write a post-processing tool and parse the generated PE EXE file and dump/convert whatever you need from there. That's actually what I do in these situations (custom binary formats).

Parsing is not difficult, since you don't have to handle weird cases or other relocation types, just throw errors if it's not what you expect (since it should be known). Should be automatically done by a build system without issues.