To what state(on/off) does the Vds figure in a MOSFET gate charge curve relate and how does the Id affect it?
how would this curve change if the Id was very low, 1mA? (For example,
  we're switching a 24V supply for a 24k Ohm load... how the total gate
  charge to switch the MOSFET will change if I'm switching a load with
  much more resistance than the R_DS_ON.


The Gate charge graphs supplied in most datasheets don't cover this scenario, so I decided to do my own tests. The answer is:- probably not much.

I tested a BS107 on a 12V supply with various loads ranging from 0.1mA to 100mA, driving the Gate with 10uA constant current to measure the accumulated charge. 

At all load currents the time for the Gate to reach +5V was 84us, showing that the same amount of charge was accumulated. However there was a noticeable change in the 'plateau' voltage that occurs due to Miller effect as the Drain voltage goes down, which ranged from 1.8V at 0.1mA to 3V at 100mA. 

Here is the scope trace for 1mA load current, showing Gate voltage plateauing just below 2V. At lower and higher currents the waveform was the same except for the plateau being at different voltages.



While the Gate voltage is plateauing the FET is operating in its linear region dissipating high power, so when switching high current you need to get through it quickly. However at low current the plateau voltage is lower and dissipation is less of a concern, so you might be able to get away with weaker and/or lower voltage Gate drive.