There are several fixed-back seats that work safely/certified/tested with a 3-point/OE belt. The Cobra Nogaro and the Recaro Pole Position ABE are the two I know of. You lose your side airbag so will need a canceler (track-hack.com) to ensure the rest of the air bags work correctly. For ride alongs, I can see this being a grey area up to the instructor/club if you only have a fixed-back seat for you. These seats allow the belt receiver to clear the side of the seat and allows the belt to sit correctly/tightly across your hips.
Have you considered just using the inertial lock to keep yourself situated? Works well and I hardly move and my seats are the base Golf ones with v. little bolstering. This is what engages when you step on your brakes hard on the street (nefore you would hit a car in front of you for example), here, you are just pre-engaging it much like is done with child seats on vehicles that don't have LATCH. The gizmos that work for child seats to maintain tension on the belt or the old CG Lock only lock the lap belt portion in, not the shoulder like this does. I see no safety downsides in my research including reaching out to HMS Motorsport and the NHTSA (they were obviously more vague but did give some clues to their opinion):
You can see how planted I stay here:
The Simpson Hybrid S is a nice piece of kit and allows you to ride along in other's vehicles that have harnesses...basically a HANS that works no matter what the configuation is and is the only one that works with a standard 3-point OE belt.
The Schroth Quck Fit Pro wasn't tested in the MK7 and has to be modified (outboard front hole has to be enlarged to fit the OE seatbelt bolt) so keep that in mind. If you spend a few minutes studying this product including the certifications/testing, you'll see they are v. specific about which vehicles it was tested in, what certifications it has, and answering the question "Can I run this in a car not listed?". The testing is for seat back strength as the harness angle down steeply to the r. seat belt latches which is typically not considered safe for any harness b/c of the stress it puts on the back of the seat...hence why the seatback test was conducted and passed for the vehicles listed as being compatible. MK5 is the last VW it was tested in. Also note these aren't typical fixed harnesses, they have a tear-away portion that allows you to move forward a bit during impact much like a 3-point does.
In the end, this is a risk-assessment discussion and we all do that differently/are willing/not willing to take certain real or percieved risks. One person will give you "OMG YOU WILL DIE" and other will tell you it's fine. Spend the time to look into these various safety bits and see what your comfortable with and do you own risk assesement. The bottom line to me is you have two ends of the spectrum, either 100% stock or 100% race car. The middle is the muddy area where you need to decide which modifications meet your risk assessment criteria and will pass the group/club/event safety checks.