You can also use <C-C> to quit command mode instead (and this one works in keymaps). It also works in Insert mode but doesn't trigger the InsertLeave event, which might upset some plugins.
As far as I know the character pending mode thing that r does is a bit unique in that it isn't insert mode, but also not quite replace or operator pending mode. If there's a way to remap any key to work with r then I'm not aware of it.
Have you considered instead remapping <Esc> on an OS level? The XKB option caps:escape turns your Caps Lock key into an additional Escape key, which is even easier to type than <C-Space>.
Additional cons: