Yes, it works now, thus I was able to test it. I picked a mage with big boobs trait and set her charisma and agility modifications to 0 in json.
And only boobs traits have the new mod field so far. So,
Lvl1: charisma 5 agility 5. Works as intended.
Lvl2: charisma 5 agility 5. Weird. Big boobs have line like "mod_stats": {"charisma": [2, 2], "agility": [-1, 2]}, so it should begin to work already since it's a second level.
Lvl3: charisma 5 agility 5. Looks like it doesn't work...
Lvl4: charisma 7 agility 5. Wtf? Agility is at its minimum, so it's not surprising that it never changed. But why she got +2 to charisma at lvl4? By level 4 she should have charisma 9 already. Mages don't have any penalties to charisma.
Even if you don't count the first level, it should be 7 at lvl3, not 4.
Then I wanted to test agility, but encountered a weird behavior.
Numbers for stats in json are supposed to be percents. If I write agility 50, it means that the character will have 50% of possible agility. So I wrote in json "agility":50 and started testing it.
Lvl 1: charisma 5 agility 12/25. Works as intended, it's about 50% indeed
Lvl 2:charisma 5 agility 20/30. Wtf? It should be 15/30 even if mod_stats doesn't work as intended.