While this isn't exactly the intended use of this mod... and there may be issues with this idea I'm not thinking of, theoretically it would be really useful for those with less powerful computers to be able to record their gameplay with the replay mod, then simply render it back out into a flawless, high resolution, high framerate video for further editing. However, in order to do this the HUD would have to be recorded and played back when enabled in spectator mode. Obviously audio etc can be recorded separately and synced with the video with very minor performance issues.
While technically possible, recording the HUD would have to be done with an approach completely different from the one the ReplayMod currently uses (especially once you add third-party mods or dare to use different screen sizes during gameplay and rendering).
Therefore we've considered such a feature out of scope of the RM. Ofc anyone else is welcome to implement it as a third-party mod (replay files are extensible, so they could even store their data in the replay file), we just don't want to have to deal with yet another recording system in the RM.