On-demand HWS can use heat-pump technology if required. Even tiny refrigerators use a heat pump. If they aren't widely used, it's because the savings are too small to warrant it.

Regarding solar HWS: The cost of heating panels, piping, and installation, has meant that they are fast disappearing from the market. It's better to add a few extra solar electric panels and use on-demand HWS, either with battery storage or via grid connect.

I've just been down this route with a new solar electric plant. I had originally assumed that Solar HWS would be included, but after doing the calcs, have abandoned the solar heating panels and instead opted for a few more solar electric panels plus extra Lithium batteries. It means I have a lot more storage that can used for many different purposes.

Refrigerators are similar to tank system, rather than on-demand hot water system. With smaller heat pump, tank is needed because it can't heat water immediately. Refrigerators have small heat pump so that's similar thing. Both should have good insulation.

Heat pump heater can be run when solar panel generates so much energy (energy price is low). That's a similar thing as batteries for energy use time shift.