I have no direct experience in modding, but I can give some analysis on existing code, and my best guesses about modding... Hopefully some of this will be useful if somebody wants to give it a try.
And from what I understand, the level of difficulty will also depend on how "clean" and "extensible" you want modding to be:
The dialog box you're interested in, is in PlayerActivate.cs, in a
long method handling all you can activate.
So I think the first step would be to isolate the creation of that dialog box in its own method so it can be overridden more easily. So that's a modification in Daggerfall Unity itself.
Code already supports using other buttons in dialog boxen, for example
MALE / FEMALE buttons during character creation.
Those are identified using an
enumeration, that is used as an index for
loading their gfxs from BUTTONS.RCI.
So, the shortest route for a mod adding custom buttons would be to provide its own BUTTONS.RCI, with standard buttons and the extra ones; And in the new dialog box code to use extra indices, say (MessageBoxButtons)21 and (MessageBoxButtons)22.
The problem with that approach is that it won't be compatible with any other mod trying to do the same thing for another dialog box, old or new.
A clean solution would be to add some mechanism for a mod to reserve extra MessageBoxButton values (or do C# enumerations provide all what's necessary already?), and then overload buttons gfx loading so that extra values can be mapped to other files. That may be doable without modifying Daggerfall Unity itself by overriding DaggerfallUI.GetTextureFromCifRci(), but it may be cleaner to, again, isolate button gfx loading in its own method.
So, for somebody with modding experience, I'd say:
- some changes to Daggerfall Unity required
- difficulty 2 (dirty implementation) to 4 (clean implementation).
So let's say difficulty 4 for the first mod that will require to mod dialog box buttons.
Until somebody comes with a better estimate...
