So for anyone who's having load errors, turns out the version provided just had a syntax error in the JSON, and the mod is actually 100% compatible.
Long story short, to get it to work, do this:
1. Open "data/world/factions/persean_league.faction" (without the quotes). This will be directly inside the Portrait Pack folder in the mods directory, so if your starsector is installed in C:\Program Files(x86), this would be "C:\Program Files (x86)\Fractal Softworks\Starsector\mods\Portrait pack\data\world\factions\persean_league.faction".
2. The first five lines should be this:
{
"portraits":{
"standard_male":[
"standard_male":[
"graphics/portraits/prt_2.png",
Simply remove one of the duplicated lines.