[ The problem I have which which-key is that it applies only after a prefix. ]
Mutating a keymap with setc[ad]r is evil! Since you bind that map to a prefix, why not use (menu-item "dummy" KEYMAP :filter FUNCTION) instead?
[ The problem with this trick is that it can be used only after a prefix. 🙃 ]

Actually, this was only the third such mode, after vi.el and vip.el, both of which were obsoleted by Viper (which itself dates back to … many years ago)