The St Martin organ at St Peter's, Petersham has two half-draw stops - a 12-17 Cornet with the 12th on the half draw, and a Fourniture IV with a 15th on half-draw. There is a notch in each drawstop - if drawn with a slight upwards pressure then then the notch will engage and hold at the halfway point, whereas slight downwards pressure allows the full draw. Sounds fiddly, but it actually works well. Initially, the builders used a spring detent to the halfway point, but fully drawing quickly through the detent resulted in a loud clunk, so they changed it in favour of the notch.
While separate stops and sliders would be preferable, and there is some compromise in the mixture scaling, I believe this approach does save space on the soundboard. I would far rather have the half-draws than not have them.
JJK