Here's one method that would work. Cut (or buy pre-cut masks if they're available) number masks from some good masking tape. I like Tamiya Yellow tape. You will need two masks for each number. You paint in reverse order. First airbrush white where the numbers appear. Once the white has cured, apply the number masks. Next airbrush black. After the black has cured, apply the second set of number masks, slightly offset from the original ones masking the white. Be sure to really press down the edges of the top mask where it crosses the edge of the underneath mask. Now paint the gray (or whatever is the main color), and after it cures, lift the masks to reveal the drop shadow numbers.
The trickiest part would be accurately cutting the masks, but by really taking your time (a magnifier light or glasses really helps) using a steel straight-edge where possible, and using a new No.11 blade, you should be able to cut some decent masks. When painting be careful to avoid paint build-up at the edges of the masks. Painting with light coats is essential.
On the other hand, at the risk of suggesting the obvious, have you looked for decals? I know that from a number of suppliers such as Gold Medal Models and White Ensign Models to name a couple do have a such decals in a variety of scales.
Andy