I noticed something interesting while idly looking through the cache drives, it seems Forza saves the last-viewed replay on the root of Y:. It's possible to simply copy and paste it into E:\UDATA\4d53006e\6442D57EA3A2, where your replay saves are, and it'll be viewable. It doesn't even have to have a particular name, which makes it very easy to rename your existing replays to make them more descriptive for managing. You can even just swap replays from console to console, they don't appear to be signed to the HDD like the main gamesave. I haven't yet tried exceeding the maximum amount of replays however. This is quite handy in a few ways, I've had times when I've gone to save a replay, and been unable to as I had the maximum amount. As long as you start to watch the replay before exiting the race, it'll be saved to Y:, meaning you can just reset, FTP it to your PC, delete a junk replay and replace it with that. Another handy consequence is for offline time trial contests, each racer could submit a best lap replay, which proves their time and legitimacy, plus if wanted it could be distributed to give other racers tips on the best line, etc. They also contain a profile name of course, so they can generally be traced to an owner.
This has all probably been found before, and it's not really a hack as such, but it's quite handy IMHO so I thought I'd post.