Welcome villait,
Nice idea, and the makings of an interesting thread. (putting my nerdy glasses on)
I'm sure it will become a possibility in the not so distant future (if it doesn't already exist).
One of the main problems would be with upstream data rates. A sling box or similar will only be uploading the required video and audio data.
My understanding is that if you want to upload a raw signal you are talking a lot of data. Any particular frequency an LNB is tuned to will be carrying enough data for multiple video/audio streams so, I would suspect that you would have an ammount of data that even the fastest internet connections (in this part of the world) would struggle to download quickly enough, let alone to think of uploading it.
I'm no sat signal expert, so if one of the more knowledgeable sat signal experts chimed in with some figures on data rates available on a sample transponder, that would be great.
The other thing would be that you would have to have exclusive access to and LNB, because it can only be tuned to one frequency at any given time.
If you were thinking of providing this as a service, you would need to have an LNB per frequency, per satellite along with servers and client side software to handle it all.
As trigger said. Let us know how you get on if you dont find a solution here.
PS: My comments are only to highlight the technical problems to see if anyone can think of a way around them, not to say its impossible.
C
EDIT:
Robbo got in ahead of me with the main problem. I should not wander off to make a cup of tea while posting.