ERC for power pin drive - Altium Designer
It seems that you have a good understanding of the current state in Altium, and its limitations with power-pin ERC.  What you state is indeed true (power pins do not have 'direction').

The only alternatives that I can think of with Altium, would be to use a hierarchical schematic setup where your power sources are all connected via sheet entries, and all power nets are configured as local.

You could then control what nets are input and output via the sheet entry properties.  You may need to disable some of the default erc checking on 'power pins', because the power nets would now be reduced to 'normal' nets.  However, errors on connection 'input' and 'output' nets would be caught using the normal mechanisms.

Another possibility is to cleverly name nets (and generous use of net-ties) to show that intended net-spokes are for input or output use.  However, ERC would not catch any errors.

These solutions seem to me more complicated than the problem.  I would recommend that you stick to the default strategy.  Connecting up a supply backwards is really easy to notice, and should be on a checklist before starting the pcb layout.