Is that what you mean by "quasi-hardware"?
Yes, a modifier that still produces a key code but also produces the keycode of the modified key, not the original. Like, pressing "fn"+"1" will give the "fn" keycode and the "f1" keycode, it doesn't give the "1" keycode. Most things will simply ignore this "fn" (or meta, sym, whatever) keycode.
A pure software modifier would give "fn" and "1" while a pure hardware modifier would simply give "F1". I thought we agreed we needed something in between, an Fn modifier that just "does the right thing".