i know this is really late but i answer the question just in case someone looks for something similar.
There is actually a meta data that can get set
i.e. like here - look for “bTrimmedInSourceImage”
// Origin within SourceImage, prior to atlasing
UPROPERTY(Category=Sprite, EditAnywhere, AdvancedDisplay, meta=(EditCondition="bTrimmedInSourceImage"))
FVector2D OriginInSourceImageBeforeTrimming;
// This texture is trimmed, consider the values above
UPROPERTY(Category=Sprite, EditAnywhere, AdvancedDisplay)
bool bTrimmedInSourceImage;
Hope that helps 