Digging a bit deeper I see that I can create materials from my padded textures and set the UV coordinates to crop the padding, but this seems like a lot of extra work for what should be a simple thing. And elsewhere I see recommendations to not use materials for UMG widgets.
Update:
It seems I can use TexturePacker to create a sprite sheet that Unreal Engine can interpret as a Paper 2D sprite sheet, and that I can set the image for my button widgets to point to the sprites in the sprite sheet. This is a reasonable workflow for me.