Is there anything in the potential = { block? If there isn't then it should show up for everybody. It's better if you move everything to allow so you can see which condition is making it unavailable for Jews.
It's most likely definition.csv. What changes did you make to it?
Can you post the modifier itself?
Here it is. It's only meant to give the bonuses to the top liege, but I don't know how to model that in-game.
Code:
imperial_rule = {
retinuesize = 500
city_vassal_tax_modifier = 0.5
castle_vassal_tax_modifier = 0.5
temple_vassal_tax_modifier = 0.5
icon = 11
}