I took a look this afternoon, and to me, it looks like Porsche went out of their way to make the input shaft seal replaceable without having to remove the transmission. If you remove the large rubber cover piece, you can very clearly see the input shaft seal. You also have very clear access to the bolts holding the retaining piece that look like they hold the input shaft in place. There are actually two retaining pieces - one is accessible through the opening that's covered by the large rubber piece, and the other is accessible through a hole in the torque tube. So, it looks like all you have to do is remove these bolts, which I'm assuming will allow you to slide the drive shaft toward the front of the car, away from the transmission, which will then expose the input shaft seal.
Has anyone recently done this job? Mine is leaking a bit, and I would like to replace it, but it isn't leaking badly enough for me to want to yank the transmission, which looks pretty brutal.