Yes, this has been discussed many times in the past. A better replay control system would definitely be beneficial, for example for server admins/race marshals who have to review races to give out penalties, especially if it was an endurance race.
As you've correctly established, rewinding per se is not possible. Instead a keyframing system to jump to certain points of the replay is the way to go.
However, Scawen's prioritization of tasks has always been a little mysterious; while this suggestion, as many others, might be useful, thought-through and at least from my point of view as programmer not too hard to implement conceptually, it already has been around for years and I expect it to stay around for quite a while before it or similarly useful suggestions (better setup organization, setup notes, ...) get implemented - if they ever get implemented, that is.