It is a brute way to declare war anyway:
1. save game, exit.
2. open file "C:\Program Files\Civilization 5\assets\UI\InGame\WorldView\TradeLogic.lua" in your favorite text editor (probably administrative rights will be needed to save it after changes).
3. Find block of code (I found it in 157-163 lines):
3. Change condition in if to guaranted falseness? e.g.:
4. load your game, now it is possible to declare war via dialog with leader. Do not now about possible consequences, probably they can be.
5. After declaring war you can return code to original state.
PS The root of the problem is in binary file ("C:\Program Files\Civilization 5\CvGameCoreDLLFinal Release.dll"), and it is hard to determine any more.