Custom status effect seems to be a no-go. I tried throwing this on Shadow Step's SkillProperties:

Consume,100,3,SevereBleeding

With this potions.txt entry:

new entry "SevereBleeding"
type "Potion"
using "_SkillBoost"
data "Duration" "1"
data "StatusEffect" "FX_GP_Status_Bleeding_A_Effect1"
data "StatusIcon" "statIcons_Bleeding"
data "Damage" "3"
data "Damage Multiplier" "100"
data "Damage Range" "1"
data "DamageType" "Shadow"
data "StackId" "SevereBleedingStatus"


And I get an icon that lasts 3 turns but doesn't actually do anything.
I tried mimicking other entries in potions.txt: changing/removing the using field, adding act and act part fields, removing/changing damageType and damage multiplier... nothing seemed to get it to actually function.