HTA 172 is extremely vague so the LEO can use his imagination. You might beat it in court but that will be after your week suspension is done with and your bike seizure and assorted fees are dealt with by you. Tough luck if you drive for a living. Unless you can prove the cop was acting in malice there's no hope for compensation. Winning doesn't carry much satisfaction.
A cop told me that the speed limit was technically what was safe for the conditions with a maximum of what was posted. In other words in an ice storm you could get a speeding ticket for doing 90 kph on the 401 in an ice storm.
That's a good point about the unavoidable inconvenience if charged.
I think the bigger question is why your butt is off the seat. There's no reason to need to do it within the speed limit, so I can see a cop having issues with this. It would be a different story if the limits were much higher, and you had to hang off in order to make the turn.
My interest in this discussion was mainly academic but I do slide my butt partially off the seat from time to time. I really so no problem with this in and of itself as long as the rider isn't being reckless. It doesn't hurt to do it (in fact it feels more balanced) and it doesn't increase risk (dangling off like a maniac is a different story). I admit, it's usually during more "spirited" riding but who isn't guilty of that occasionally.