YOU NEED TO TAKE A DRYSUIT CLASS.
The mere fact that you asked the question, and then got an off-the-wall answer about how to counteract an inverted, feet-up position in a drysuit, beggs you to take a drysuit class.
Followed by a grand set of reasons (in terms of technique recommendations) that illustrate exactly how NOT to dive a drysuit!
First, what is a drysuit for? It is for exposure protection, right?
What is your BC for? It is for buoyancy compensation, right?
Your drysuit is designed to have gas added to (or released from) it SOLELY for the function of protecting you from exposure. That is, its reason for having an inflator is because it MUST in order to do its job - otherwise you'd get squeezed to the bejeebies.
Your BC, on the other hand, has multiple dumps (for different positions) and is designed to be used to control your buoyancy and position in the water.
They are very different functions. Perverting one to serve the other's purpose is unwise at best.
For your suit, you want the dump valve FULL OPEN all the time. If you add too much air to the suit with it set full open, in a normal swimming position or a feet-down position, it will harmlessly bubble out of the open dump. No harm, no foul.
This makes it somewhat difficult to get into a feetfirst uncontrolled ascent (and thus need to do the "tuck and roll" deal.) Why? Because in order for that to happen, you must have an abundance of air in the suit, AND you must also be rising. Why would you ever be in that position voluntarily? I can't think of a good reason to do so.
Ok. Now that we've settled this, we can dive.
If you are properly weighted, you should be negative by the weight of your gas ONLY on the surface originally, with all "extra" gas out of the drysuit.
To decend, you partially dump your BC. You are now, for a single tank, -2 or so. You will begin to sink at a controlled rate. As you do, your drysuit will compress. You compensate by taking off the squeeze of the suit. If you overcompensate, the extra gas vents from the open dump. Your decent will be quite controlled, since unlike a wetsuit you do not gain negative buoyancy as you decend, as your suit's volume (as you unsqueeze it) remains a constant. The entire decent, you are only -2 or thereabouts, and as a consequence your decent is quite orderly and controlled. You DO NOT compensate for the decent with the BC - you do that with the suit, since the suit is the major component of your buoyancy that is changing as it squeezes you.
As you approach your desired depth, you stop your decent with the BC. This will require very little air.
When it is time to ascend, do so either horizontally with the arm with the dump (normally the left arm) slightly elevated, or, if you prefer, more upright. If you take your BC control in your left hand, you will naturally elevate the shoulder slightly, which is perfect for automatic dumping of the air from the suit. Control your ascent with the BC deflation control as usual. The suit will take care of itself, since you have set the valve full-open - as you ascend, the extra air will rise to the highest point, which is your arm, and be vented to the surrounding air. If you make a normal controlled ascent, there will be a continuous small flow of bubbles while you are doing so from the deflator valve on the suit.
Very simple and controlled.
I dive for lobster in my drysuit in the wintertime around here, and have no problem with going inverted. Then again, there isn't much air in my suit, so there is no way for it to pop my fins off and turn me into a rocketship.
BTW, if you find yourself with a need to dump air FAST from a drysuit, you can as long as you are upright. Stick your finger down the neck seal and break it loose. You are warned, however, that doing so will intentionally flood the suit at the same time it releases the air - if the water is cold (and why would you be wearing the suit if it was not?) you're gonna get a nice big surprise!
