It occured to me that diplomacy is one of the most important missions in the game, second only to sabotage in my opinion. It also appears that, next to "Jedi Training" and "Recruitment", it is the only mission that you can't give to special forces. To that end, is there some way to allow special forces to use Diplomacy missions?

I don't think so. In RebEd, you can replace a Spec Force with a created one, but you have to base it on one of the previous ones (I think). Since none of the Spec Forces have an initial diplomacy skill, I do not think you can add it. I'll have to give it a whirl and find out to be sure.

It is beyond RebEd, but I think it is possible as each mission type is allowed for major and minor characters (I think) and separately for the special force "types". Rebed actually changes that "type" for the spec forces, but since none of them is originally capable of diplomacy missions, it cannot help you.

Also, as I remember the spec forces can have diplomacy value, but I do not know whether it can be set with RebEd.

Maybe I will be able to be more specific if I can find the source code of the program that was designed to allow the assasination mission for alliance characters. (Well, and gain some feedback whether it actually works or not ...)