I ran probably 25-30 gallons of used tranny fluid through my 99' with VP44 injection pump, no ill effects. I never ran more than 3-4 quarts mixed with a full tank of #2 at any one time but I did burn quite a bit over the space of a few years. I also burned several gallons of used engine oil but never more than 2 gallons of used motor oil mixed with a full tank of #2.
Now that I have a 12 valve for a daily driver, I run much richer mixtures of used oil, used tranny fluid, and used gear oil. The 12 valve injection pump seems to be robust when it comes to mixed fuels.
I don't purposely seek out used fluid to run as fuel, I simply dispose of my used atf and oil changes by running it in the tank.
I don't think I'd feel comfortable purposely obtaining used atf for the specific purpose of running it as a fuel but a quart or two once in a while seemed to have zero impact on my 99'.