Mortise lock conversion from key to privacy?
I've done that for a bathroom recently. I replaced the locks of several doors entirely with this lock (fit like a charm). For the thumbturn, I purchased this plate and thumbturn kit. It required a bit of 'macgyvering' but it turned out well. 

I had to combine the thumbturn provided in that kit with the key provided by the lock. The thumbturn provides a handle, a square 'arm' with a hole in it, and a pin which connects the thumbturn and the 'arm'. Luckily the shaft of the key fits in the hole of the thumbturn; by cutting off the handle of the key (to appropriate length) and drilling a hole in the shaft you essentially swap the 'arm' provided with a functioning key. Photo illustrates the result: 
 

The pin portion of this thumbturn contraption is hidden behind the plate provided with the thumb turn. Result works and looks great.