I've had this issue too. I really is hard to understand why quite so much water gathers in the boot. I could understand a cupful, but at its worst, I think I would be looking at nearly 2 litres.
If you wanted 2 litres of water in the boot lid, I don't think any method you could choose would be nearly as effective as one that circumstance creates on the A8 D2!
Can you still buy the grommets?

Grommets are moulded onto the tubing. :(
(Edit: Can be split/peeled off so maybe it's just glued)

Had this issue with my S4, was the cable gromit to blame as it enters the boot lid. Resealed that and all dry +++

Thanks all for the info. I will attempt to seal this before our next torrential downpour.

My boot grommets aren't in particularly good shape, but my boot isn't leaking (yet).
So I've just bought some of this to experiment with!

I just used some (Sugru) on one of my grommets, see this thread: http://forum.a8parts.co.uk/showthread.php?t=11483 ;)

Thinking this through a little, I wonder if the boot interior is actually at a little bit of low pressure, due to the A8 aerodynamics?

It's quite possible that a low pressure area forms behind the car at speed, causing air inside the boot to leak out- lowering the pressure within the boot and sucking water in through any available wet area.

I know mine was the grommet under the leading edge of the boot door, but why water would choose to go uphill into the boot door, when it could go down into the gutter is less clear.