Not feasible right now, as far as I know. I hope a more powerful keybindings system will be added in the future, though.
But you could approach this differently if you're willing to put up with some frustration:
Forget about command mode and open an input_panel instead. Type the command there and parse that. I won't work well for motions, and it's far from ideal, but it's something at least.
On the other hand, simple motions should work well with the existing support for command mode. hjkl, HJKL, b, e, a, E, A, o, O, G, gg, etc...