I'm not sure that i see the point... Let me see if i understand what you're proposing:
You happen upon a question, and reserve the first spot. Then begin writing up an answer. You have some idea of what the answer is, but need to test your idea, so it takes you a while...
Then i happen upon a question. I know the answer, type it up, post it, and it shows up - first and only answer.
Ten minutes later, the OP hasn't returned to accept or dispute my answer, and no one cares enough to upvote it. You, having finished testing your answer, post it. It shows up above mine. They're both good answers, but now yours sorts first. Eventually, the OP returns, sees yours first, confirms it's correct, and accepts it.
I just wasted my time.
Alternately, if someone had up-voted my answer prior to you completing your answer... or the OP had shown up and accepted it... i'd have held the top spot anyway under the default sort conditions. Same situation we have now.
Alternately, if i'd rushed to post a quick answer, and posted a lousy explanation as a result, the OP might have disputed it, down-voted it, or otherwise dismissed it... and then read and accepted yours. Same situation we have now.
Finally, if i'd rushed a quick and dirty answer in, but immediately edited it to add more info / a better explanation / snazzy drawings... and took only 4 minutes 59 seconds to do this... But you'd finished your answer in less time... Then you'd hold the top spot, while my gaming would have been in vain.
IMHO, this is the only scenario where what you propose holds potential for some improvement over what we have now.
So, we get one scenario where the new system is arguably worse than the existing system, and one where it's an improvement. Pretty much a wash, IMHO.
There's been a re-occurring suggestion for "... is writing an answer" notifications. I suspect this would have the same potential advantage (allowing users to avoid stepping on each other's toes), without either the complexity or the potential downside of this one. Of course, there are other downsides...