So, if HOMER actually takes the amount of hydrogen produced into consideration (I doubt this is the case) depending on the % of rated power, then I would say you could go all the way down to 10 or 15% for the min load ratio.

HOMER assumes a constant efficiency, but not a constant output, if that's what you mean.  So it assumes an electrolyzer running at half throttle would produce half as much hydrogen as the same electrolyzer running at full throttle.