When you're hanging upside-down on a Screaming Squirrel you're experiencing -1G. It's the same force feeling you'd get on one of Skyrush's-1G hills. The reason Skyrush's hill 'feels' more forceful is all those rate-of-change things we've talked about in great length before.
If you want an inverted 0G hill, then you just do what B&M do on their hypers but flip the track. It's pretty simple, really. If you want to give that feeling of being dumped out the seat, you just draw the hill out a bit more and target the 0.2G hill (and flip the track). You can also vary the hill profile to give you that little pop right at the top.
That said, I'd argue that any G-forces under the -0.5G mark is going to feel a lot more like hangtime - especially if you've got the visual cues of being upside-down. What you've drawn would be something awful like -3G and be foul.
And no, I'm not sure anyone has ever made anything like that before.