It wont be available in race-mode, only in practice mode. And you wont need to quit out, you just select "Restart" and you'll be dropped back at the point you chose when you specified your "starting point". ET will be automatically recorded for the sector, and u can compare your times with previous times youve done for that sector.
Well how do you know what corner you need to practice and from where to where? You must first race that track right? So the choice is: 1 Race the track, find a difficult spot, rewind with perfect accuracy to the point you want to try again from (because in rewind there is no need to find a point on a map or know meters in, you literally see down to the inch where you are and have the benefit of seeing from car view for better accuracy) and try immediately or 2: Race the track, note the corner that is hard, either quit out and go try with your method (trying to figure the point on the map you want to consider the start and end) or continue through and take notes on each corner you want to practice later, then quit out, go to this practice mode, set up the chunks you want to test and then try them out.
As for saving times for a section, you are solving a problem that isn't there. No you can't save times for sections with rewind, but it still gets you the practice you need to get through the corner decently.
I don't think having what you suggest is a bad idea, why not practice sections of the track with a timer, but I am saying it's neither a less cumbersome method to do what rewind does nor as well integrated into the game.
Since you will be entering the track some distance prior to the timer-start point, you'll be able to take your preferred driving line before you get to where the timer starts.
Also, if you're doing a standing start you can choose your lateral position on the track.
Now you are getting yet more complex and move farther away from "not cumersome" and again, why do it all this difficult way when you literally can hold a button until you are where you want to be and you nkow the speed, angle and location is EXACTLY what you did coming into this corner?
It's very simple, but with rewind I dont see how youre gonna be able to record your time for a specific sector, unless u do it manually.
You won't be able to, and if that's important to you, then you would have to do something else. But the point is even without recording your time for a sector, rewind gives you a great tool to learn a section through practice, immediately, any time, with little setup integrated right into the race environment. You just go race and you dont' have to worry about remembering which corners you want to practice and go set them up, you just hit rewind and do it.
Your idea isn't a bad feature, I think it could be cool to have also, but it doesn't do what rewind does better faster or easier... it just does something slightly similar as well as something else (record time for sector) in a very different way.
Basically if I had to choose one or the other as an educational tool, I would choose rewind. If both were offered, I think your idea would be a great thing to have in also.