Obviously the definitive answer would come from building such a keyboard and doing suitable tests. I don't believe that comparisons between conventional keyboards and either touchscreen or Surface-type touch keyboards are necessarily instructive here, for two reasons:
- If all the keys other than the spacebar can be distinguished by touch, then the main disadvantage of touchscreens (the cognitive effort to acquire the keys) is not an issue; you know your thumb is in the right place. Actually, even with pure touchscreens, the spacebar is likely the least problematic key since its size and position at the edge of the screen makes it easier to acquire.
- The way people tend to operate the spacebar, using their thumbs as hammers, is different to how they operate the other keys (position finger, then press until you feel the click). This would be different in applications involving multiple presses of the spacebar, but that doesn't come up much unless, like me, you have the bad habit of two spaces to begin a sentence. It would also be different for someone who used their thumb above the bottom row, but I don't think anyone with ten working fingers types like that.
Qualitatively, though, I imagine it to feel unsatisfying, or at least take some getting used to-- if you think of typing as playing a percussive instrument, it's a very different drum kit. You only have to listen to someone typing to hear how they use a satisfying whack on the spacebar to celebrate a word well typed...