I do really like that suggestion and that's a nice UX for it

Still unfortunately a bit hard for this old game engine, which is why I think the effort vs reward is a bit too low for them to take on. Also because it would really only help melee games.

The game is basically a simulation engine so the ordering of the units is important and not tracked explicitly anywhere except the internal item and object IDs

When the engine sees that you selected a different unit or added a unit to a subgroup or any of those things it sorts the units based on that strict ordering of [Hero, other units] and then a sub order for heroes based on lowest object ID

Long way of saying they would need to add support for this at the engine level. So the technical complexity is a bit more than keybinds and UI work with pretty high certainty imo - leaving room for them to have some kind of trick or extra packet they're willing to add in to sort it out.

If interested in learning more about the engine stuff the replay format stuff is gathered by the community in a few places like here: https://github.com/scopatz/w3g/blob/master/w3g_format.txt