Out in Nevada, the Nevada Northern used that method to oil the air compressors. The first time I went out there to help them get the NN 40 running I decided to fill the air pump lubricator while we were steaming he up - only there wasn't one. It was quite baffling until I found a line running from the mechanical lubricator for the cylinders and valves off toward the air pump. Yes, while on spot, you had to remember to go out and crank the lubricator every once in a while. It seemed to me to be an odd set up. When rolling down the track at speed, the pump would drown in valve oil, but while creeping down some hill using gobs of air, the pump wouldn't get enough oil.
When I worked for the Ft Worth & Western we had a sexy air pump lubricator set up that used a small air piston plumbed into the bottom of the low pressure air side of the pump. The piston conncected to the crank on the lubricator. They had a two compartment lubricator. One side was filled with valve oil for the steam side, the other side was filled with air cup oil. It did a real neat job. You filled it once a day, it ran the same speed as the pump. I don't know where the piston and cylinder came from, but I'm sure it was an off the shelf item from McMaster Carr or something. There was a spare on the parts shelf.
WABCO did make a mechanical lubricator for air compressors, but I've never seen one used, so I can't comment on them.