This borrows from @jola5's "Not quite neo" code. This allows me to maintain blocks of keymaps in the userspace, so that I can modify the userspace, and this is reflected in all of the keyboards that use it, at once.
This makes adding tap/hold mods, or other special keycodes or functions to all keyboards super easy, as it's done to all of them at once.
The caveat here is that the keymap needs a processor/wrapper, as it doesn't like the substitutions. However, this is as simple as just pushing it through a define. For instance:
#define LAYOUT_ergodox_wrapper(...) LAYOUT_ergodox(__VA_ARGS__)
Once that's been done and you've switched the keymaps to use the "wrapper", it will read the substitution blocks just fine.