I've been using Canon gear the last six years & I don't know if this is currently possible in any camera body. If not, I believe it would prove highly helpful for working professionals.
When working in time-sensitive situations, I find myself frequently forced to use Av mode when I would prefer to remain in Manual exposure. Additionally, I find myself slowed down within Manual exposure mode when moving between inddor and outdoor light and/or other situations of dramatic ambient light fluctuation.
My wish would be for a menu assignment function for the "multi-function" button (next to the shutter release). Pressing the button while in M exposure mode would then set the shutter speed (like in Av mode--and another option could emulate Tv mode), and releasing the button would resume normal M exposure mode usage.
Personally, this is my biggest wish for the 5D3, and I would LOVE to see it implemented. (My second wish would be for the flash exposure lock to set the exposure of Flashes which are otherwise set to manual flash exposure, but that's another story).