I tested using F6 while sitting in a vehicle to lower exertion and it wasn't very effective at all. I wouldn't suggest it as a viable solution. You will have to either bring a chair with you or make a chair where you intend to rest until the rest option is added to the vehicle menu. I like Abade's suggestion: Yeah, this seems like more of a natural evolution to the rest command. Let us rest anywhere and if I may add let us sleep anywhere as well.  Give us different bonuses for the quality