Definitely have not seen a tweak like that myself, sorry. It would probably be a lot easier to simply rethink your in-car setup, no? Is yours built-in, or aftermarket? (For instance, maybe you could switch to something that uses the 30-pin connector instead of a headphone jack.)
I think I found what you want.
English is not my mother tongue, so plz forgive me if I didn't understand correctly.
The tweak is "Upside-down" (it has updated recently)!!!
Try to install the tweak, and turn "April fool mood" OFF in Settings!!
In my case, this is almost completely working by using SBRotator with it, except that sometimes Zephyr and Notification Center doesn't work.
When I using my iPhone upside-down and try to close app by using Zephyr jesture, Notification Center appears.
Some times the opposite occurs.
But there is no problem when I using my iPhone in a usual way.
So far, I'm quite satisfied !
Try it ;-)