I used a one gallon garden sprayer (Spectracide #27019 at Lowes (about $13USD)), added a chuck to use a compressor to pressurize it (optional (note do NOT use over 20PSI)). A 90 degree 1/4 turn water valve ($4USD) with a 3/8" compression fitting on one side and a 3/8" hose fitting on the other side. Removed the sprayer valve, and replaced it with the water valve, added the tire valve and it works great (and it was much easier than I thought it would be).
Throughput is about 1 liter / minute.
I have not found a commercial tool that does this for civilian use. I put a 2" piece of 3/8" clear tubing ($0.25/ft) over the hose fitting side of the valve just for a little extra reach (I needed a 6" piece for the front diff) adjust to your own needs. And a hose clamp on the hose end for security ($0.50USD).
Step 1: Add the Tire Valve
Make sure the edge of the gasket will fit up through the hole for a good seal.
You might need a rat tail file to clean up the hole.
These are the 0.45" (to 0.625) valves at Napa and Advanced Auto Parts.
I guess you could use a 1/2" drill bit, but I liked the tight fit of the 29/64" bit.
Step 2: Remove Hand Sprayer Valve
The compression fitting worked great on this particular sprayer Spectracide #27019 brand 1 gal sprayer.
I added a small hose clamp because of the pressure (note do not use more than 20lbs).
The build in pump should work also, but I did not test that as it was not the design, and might take a while.
The dip tube has a filter screen on it, I left it there (can not hurt), but it does not allow total fluid extraction.
You could cut the filter off and the tip will rest in one of the reservoirs that make up the 'feet' of the bottle.
You will need to note (and can reach in with a screwdriver and adjust) where the pickup is, so you can tilt the bottle that way to maximize fluid exaction.
Step 3: Final Pictures
I did not take more pictures because I just built it, and then decided it would be useful for others and kind of retroactively documented it (but it is pretty easy).
I am currently building a 2 gallon unit for transmission fluid.
And have already modified a unit for brake fluid (just added the air chuck to the Bavarian Autosport brake bleeding kit) (got tired of pumping up the tank).
Just finished 3 diffs in a row and it worked perfectly!