I’m preparing an edition of something for a student, and I wanted to nestle the fingerings into the stave closer to the notehead, and was surprised to discover that I couldn’t whiteout the background of the fingering numbers. I would love to see this added eventually.
… at the moment you can turn them orange, at least 
Dorico will only white out the background of fingerings that it thinks are supposed to be positioned inside the staff. You can tell Dorico to position a fingering inside the staff via the Properties panel, instead of dragging it in Engrave mode.
As a workaround you can set the fingering to “Position left of notehead” in the “Fingering and Positions” property and then move it it wherever you want with a whiteout background.
Thank you both. It’s the “left of note head” property that I needed. It didn’t occur to me to try it since I wanted the fingering above the note, and there are no other properties that would make it obvious. Much obliged.