Indeed, H/P thresholds are usually not that high, and if you follow an army around for a few campaigns you'll easily satisfy most requirements.
Though in this case, Marshal Formations are already tied to leadership, and we should probably keep it that way, even if to just remember that the leadership skill has *some* use. 
I agree it on all counts, as not only is it relatively easy for your char to reach any required H/P thresholds by campaigning etc, but doing so also allows "yourself" gain the RL experience of how best to deploy armies in battle etc.
The most simple and practical solution, and one that could only be argued against by those who wave the equality for all banner, (in other words want every feature available to their chars right from the off) would be to have the custom settings option added as the one a marshal gains after attaining the last of the others.
