Imagine I took that sock & velcro, packaged it up, and sold it at Babies R Us for $15. Would we say it's an aftermarket product and not recommend it?
Absolutely I would say it's an aftermarket product. I also say that a toilet paper tube is an aftermarket product, and don't recommend it. Then again, twisting the belt is allowed up to three twists, but only as many as are necessary to get a secure install. So if you give your female buckle an extra half twist to hide the button, and it wasn't necessary to get the seat tight, technically that's wrong to do. Also, having the button on the inside could confuse a first responder (professional or good Samaritan) for that critical moment, or nearly completely obstruct its use. For example, having the button on the inside with the TrueFit in my car is a bad idea. I did it once, and it took me about 5 minutes to weasel my finger in there and press the button.
When there's a situation with no clear-cut answers, it's the parent's decision about what to do. The parent has to weigh the risks of a potentially unbuckled seatbelt from the passenger next to the seat, versus adding an aftermarket product or giving an unnecessary half-twist to the buckle.
In my car, with the Radian, I did an extra half twist of the buckle. It was not only passengers that I needed to worry about, but DD liked to kick the buckle and I was afraid she would release it herself. After the aforementioned incident with the TrueFit, I realized that the TF is too wide for anyone to sit next to in my car, and DD can't reach the buckle, so I left it on the outside. I had initially hidden it on the inside out of habit, because that's what I did with the Radian.