I've seen the waist d-ring secured with a non-toothed retainer - allowing the d-ring to be re-positioned during the dive. As the tanks get more buoyant, just slide the d-ring forwards on the waist strap. It worked well.
That would suggest entry without the tank and clipping on in the water. Or else the tank may shift upon entry and they'd have to readjust on the surface prior to descending. Not a bad solution but it would also mean repositioning for the second and any subsequent dives. If the D-ring is fixed with a serrated tri-slider it should prevent slipping so two D-rings would be ideal and thus no repositioning of the D-ring during and after each dive. I would be interested in hearing of any other solutions to this issue. Personally, I strap a weight to the tank before diving but I know others who don't like to do this.