I think the grounding lug is on the block beneath the alternator. At least, there is an otherwise unused stud down there, and that's what I used it for.
Yes. The grounding lug is any point on the block where you can get good contact and easy access. I'm not aware of there being a designated lug on the A4.
mine is grounded at one of the bolts holding down the transmission access plate at the rear of the engine. As Kerry mentions, pretty well any substantial bolt with good contact will do. Painted surfaces and those heavily sludged should be avoided (or cleaned to bare metal)