Very cool and I like the idea of a "meta-assembler." The most-recent version of flatassembler (fasm 2) is built with fasmg which is also a "meta-assembler" of sorts, in that it also doesn't directly support a specific instruction set and instead is a very powerful macro assembler. I'm keen to check out functionality overlaps between the two implementations.
https://board.flatassembler.net/topic.php?t=19389
https://flatassembler.net/download.php