So I went to Lowe's and dug through the plumbing aisles until I found two cheap pieces:
a 1.5" cleanout plug and a 2" x 1-1/2" bushing (threaded)
(assembled on the left, pieces on the right)

Screw the cleanout plug into the bushing and you have yourself a nice compact grease cap installation tool, for the pricey sum of $1.70.
Should work for most 2000-3500lb rated hubs (with the 1.98" dust caps, which I believe is standard for small trailers).