Related discussion: Feedback on feature (and status of lld linker script support?) - #7 by smithp35
I’d hope that your needs can be satisfied without adding linker script support to the Mach-O support.
One possibility is whether you can use the ELF port and then transform the ELF ET_EXEC/ET_DYN file to a Mach-O file. See Plan 9 a.out executables with lld - #4 by smithp35 for a similar suggestion for a.out
While ELF is mostly a super set of a.out, Mach-O has many features not in ELF. You may need some placeholders in the linker script (e.g. . += 16 to reserve 16 bytes) then use a post-processing tool to fill the content.