Is there any chance you could add a 1.25u r3 tab? This enables a majority of 40% keyboards (with proper rows/keysize atleast) with only 1 key.
Some keys that could be removed for this key: r1 end, r1 pg dn, r3/r4 pgup/dn or r2/r3 pgup/dn (why cater to 2 subniches of 65% when you could add 1 key for 40% compat?), accent colored numpad enter, or mod colored ` key.