How to access and change UI Brush from outside BP?

So I’m trying to do a UI based transition to segue between a few dj scenes. I’ve gotten as far as creating a UI material that uses an alpha channel as opacity. I open the UI on level begin play and use a timeline to lerp my alpha channel between 1 and 0 for a fade in transition, and do the opposite for a fade out transition on an action input. All that works fine and dandy.

I want to have different images for going, for instance from earth to space, and vice versa. I made a struct with 2 slate brush parameters. Then I break that contruct and set the brush based on an integer. That is where I am stuck. I can’t figure out how to change that integer (just needs to be 0 or 1 at this point) variable. My first instinct was to change it from the level blueprint but that doesn’t seem to be working out. I’ve got a material collection parameter value (transitionnum) that I can set in my level blueprint, but then I don’t know how to access that in my UI BP. Seems like this shouldn’t be too crazy. Let me know if i’m missing anything obvious.