So I ended up buying 3 of these, under the assumption they will work for horizontal dropouts, and the short answer is, "no, they don't really work for horizontal dropouts".
As far as I know they are designed for track ends, but *can* be used for horizontal drop outs, provided the axle is long enough. My (solid) axles were not.
As they are designed for track ends, to get them to work for horizontal dropouts, you need to put them backwards, as per the attached pictures.
As they are installed backwards, the thickness of the chain tensioner itself means the axle nut cannot be fully threaded onto the axle (as, I said my axles were not long enough - the threaded section of the axle protudes 16mm from outside face of drop out).
So don't get this if you have horizontal drop outs and a length of axle that means the threaded section does not protrude from the outside face of the horizontal drop out by significantly more than 16mm.
The Genetic Chain Tug looks more promising, as the thickness of the chain tensioner isn't limited by the bolt diameter; although I haven't tried this yet.