It is different from other MP styles so it may not require the 8 player max.
So my idea is that if you have your scenario don and ready to play... You can load it in hotseat and play yourself. Within one turn or two you should be able to get the diplomacy right as you will or should have the option to approve all your own treaties.
This would depend on the fact that you can take players and turn them from users to AI but that should be possible.
This would give you a save game for the civ with all diplomacy set. You would have to unfortunately download a different file for each civ you wanted to play as however Save game files seem to load 10 times faster than .bic files.
Even if it is limited to 8 that would still allow you to make diplomacy in 8 player scenarios.
If it were originally loaded as a hotseat game, would it not load as a hotseat game after you saved? Nice idea but I don't think it is going to work.
As for your statement about saves loading faster than bics, I agree but your 10X factor is a bit off - more like 30 -50X especially on large complex scenarios. I made one that takes 30 minutes to load from a bic and less than a minute for a save made after the inital load.
As I said it all depends pn being able to conver a player to an AI. So if it is not there originally and they don't give us a diplomacy editor in a patch, the could at least patch it to give us this option.
I've never seen that ability in any hotseat game, one human player quits and the other wins. I would like to see what you envision, but I'm not holding my breath.