Wizkids Rules!

Boomerang: Action dice with boomerang are re-rolled after they’re used. If they reveal an action face, add them to your Prep Area.

Spark: While a character with a Spark ability is active, if you roll a [BURST] face on one or more dice, use the Spark ability at the end of the current Step. During the Roll and Reroll Step, only check at the end of the Step.

After using an action with Boomerang, if it rolls on face showing a burst would this trigger Spark abilities?

Based on recent rulings concerning Energize and the Used pile, it seems that Boomerang should NOT trigger spark. Once used that action would be in used (out of play) which is an "unrolled" state. Thus the rolled face wouldn't exist other than checking to see if Boomerang works. Is this a correct interpretation of previous interactions?

No, the reroll from Boomerang will not trigger Spark.

As you’ve correctly pointed out, the Action die with Boomerang is in the Used Pile (an unrolled die play area) where such abilities based on the result of a roll will not trigger, unless specifically stated in the ability.