Updating a texture region using a mask on the sourced texture

This is probably a long shot but has anyone figured out an easy way to do this?

Allow me to go into a bit more detail:

I currently have a transient dynamic texture (let’s call it A). I want to update a part of it using only the non transparent part of another texture (B). Is there some way I can update a region in A using only selected pixels from B? I’m currently using UTexture2D::UpdateTextureRegions and looking around there doesn’t seem to be much in the way of support for this, all pixels in the designated region in A will get overridden by the data in B.

I would use decals on top of a large plane rather than this but my usage case would require about a hundred thousand of them in the long term so I would prefer to use dynamically updated textures.