I've used a conventional single point sling for kneelers since 2004. As the sling cannot be attached to the jacket it can be moved up and down my upper arm to alter the effective length of the sling to allow similar sling tensions at various target angles.
I've never really tried it for standers, but I hardly ever miss those so I don't really want to change anything there.
I started using the sling for kneelers as I totally lost confidence in what I was doing. I'm not sure just how much difference it makes now; I practiced before the world champs in Italy without the sling, as I understood that they were to be banned. I couldn't see a lot of difference, my hold pattern looked identical with and without the sling but perhaps everything moved a little faster without the sling.
As with so much of FT the open rules allow the competitor to use their creativity to find an advantage that works for them; and these fixes need not cost a lot of money. My first single point sling was the strap for a sports bag I was given at a garage after buying a tank of fuel!