CA have at some point in their recent patching either fixed an error or removed a feature in their anti-piracy system that corrupted saves of editted startpos ESF's.
The old bug meant that even the workable solution to making clans accessible made the campaigns unplayable due to the inability to continue play after you exited the game.
(Thanks to Kamos for providing an updated link after Megaupload shut down)
The mod is 270mb, this is unavoidable due to the amount of ESF's I had to create.
(Almost 1gig uncompressed)
Extract the .zip to the folder that has your Shogun2.exe.
C:\Program Files (x86)\Steam\steamapps\common\total war shogun 2
Run the AllClansPlayable.exe to start the launcher.
(For anyone having problems make sure the AllClansPlayable.exe and the AllClans folder are in the same folder as your shogun2.exe!)
Any problems, feel free to ask in this thread.