TL;DR: Any boss that doesn't drop equipment (parts of character set) would provide nothing as bonus loot since it only affects equipment
to understand this we first must understand what does this boosted, whenever you get a boosted or prey bonus in a boss it will roll an extra equipment loot (based on the bonus chance), what does this mean? it means that you will have extra chance to obtain another roll for parts of equipment (weapon, armor, legs, necklace, ring, helmet, shield, quiver or boots and according to This answer pocket slot and backpack too) nothing else, so basically whichever boss that doesn't drop a part of those would literally drop nothing but extra points as bonus, an example of this behaviour was tentugly, it went boosted few days after the patch but as doesn't drop any part of set it only gave extra points.
About proofs, check this quote from Boss Up Teaser
With every server save, a randomly drawn boss of the Archfoe category will receive a temporary boost until next server save. During that time, slaying the selected Archfoe will count as three kills for your Bosstiary and the boss will also carry more equipment loot than usual. This boost will always be higher than the maximum equipment loot bonus which can be achieved through the Bosstiary.
and this image from prey bonus (which is replaced by boosted bonus since they affect exactly the same):