GUI is easier to fake than anything else in a video, so I don't see why it'd be required to create authentic evidence. Was that discussion on the forums? If so, could you link it so I can read it?
I don't see how you could use a video/replay for appealing at all and if any server accepts such method as a way to prove your innocence their system is majorly flawed imho.
Adding the chat and scoreboard GUI shouldn't be difficult. However, unless that's the only reason a major server doesn't accept it as evidence for reports, I don't think it's worth adding.
Adding GUI Visibility options was rather a personal solution which I thought of since that is what is used to recognise the environment of the server. This could be used to prove the legitimacy of the server, identifying whether disproved teaming occurred on the Solo Games, or may be used to identify the date of when the Replay was recorded.
Lets put it this way. If you record evidence of someone hacking in F1, there isn't too many factors which could account for the legitimacy of the evidence. However without F1, you will have scoreboards, chat, and maybe even (not really recommended nor needed) Boss Health (used mostly for server advertising), which are several ways that the evidence can be viable in proving that the conditions are met to punish the rule-breaker.
And I have only been discussing this with friends and Mineplex staff through TeamSpeak, both within chat and voice communication. However, if further implementations do occur, I hope to bring up this change with Mineplex specifically, and see if they can consider ReplayMod as a viable alternative evidence collector for Appeals and Reports.
I would personally like this to be a toggle-able feature too but for other reasons.
The reality is that if someone wants to forge evidence, they would go to great lengths to do it. Having the chat and scoreboard visible would not make it any harder for them to forge evidence, however I do see their point of view in that it would be easier to accept evidence captured using the Replay Mod if you could see scoreboards etc. I know on the Hypixel network they usually put the date in the scoreboard, and while this could very easily be replicated by someone that wants to forge evidence, it still goes a long way in preventing players getting banned for the same offence multiple times.
I can see why this feature would be low priority, but if it is easy to implement then it would just add value to the mod for those that want such a feature :) Just my two cents.
I wouldn't, however, want the developers to take time away from more important features and/or fixes just to add this.
You put this very well, and I am glad you were able to post this because this is the type of reasoning I was trying to describe, however as you stated, it shouldn't be highly prioritized. If it becomes a hassle to implement into the ReplayMod, it is best to maybe think of another implementation to make this mod a viable tool for evidence collecting, since then again, that isn't the mods focus.
All I ask is that the Developers take an attempt in adding this is, and seeing how it goes, and hopefully it will be featured!