I was curious if Jarate covers enemies who are under Ubercharge. Has anyone done some experimentation with this and if so, does this also apply to the other 3 Mediguns' Ubers?
No, it doesn't. Ubered players are not affected by status effects. Note that this only works with the stock Ubercharge.
In fact, flashing someone with a stock Medigun's Ubercharge will clear all status effects currently on them. Just be aware that the amount of time on the Uber goes down every time you change targets, to a minimum of 2.666 seconds (from the normal 8 seconds) for 5 or more targets.
Note that this also only works with the stock Ubercharge.
Having said that, all Mediguns will shorten the duration of the fire, Jarate, and bleed status effects simply by healing said person.