I am assuming from what has already been said that the suggestion is Anglo/duet straps on English rather than the English thumb straps would be totally impractical and ridiculous. It should be mechanically possible to attach handstraps to some part of the concertina without necessitating the hand-bar, but as someone has already said the extra need to move rapidly from one button to another on the English when playing complex melodies needs the 3 fingers in use to be as balanced and free as possible. Playing chorded classical pieces and accompanying song on the larger heavier Englishes has benefited from hand straps.

Rethinking my initial statement, I've just thought of an exception. I have a miniature Anglo with no straps at all. It was obviously built to just grip the ends with thumb and little finger which is how I played it to start with. I found that occasionally with a little sweat involved my grip would slip off and lose control. I solved this by attaching 4 rings of velcro for both little fingers and thumbs, so it is similar to the way an English would be held. Obviously this would not be possible with a normal size Anglo.