As for 1.7.10:
- The mod is using some nifty technology which will make it hard to backport it
- It is probably incompatible with most mod packs seeing as it already doesn't work with quite a few mods on their own
- You probably don't want to use the ReplayMod to record a let's play series (well, not exclusively) as it doesn't show any HUD
Not sure what you mean by pause/play buttons for keyframe selections. There is a path preview button which you can use to preview the path.
Toggle Chat: That's in the next release (1.0.4). It doesn't get rendered though because that'd probably look pretty stupid
3D text or image: CrushedPixel planned to allow importing of .obj (or similar) files which would allow that (he's currently on vacation though, so that might take some time)
Reverse replay: This is extremely hard to implement and will never look like simply reversing the rendered video