I normally don't worry about it, but if I have a guest and they see me pottying him at the sink I do tend to feel embarrassed. Likewise, at my in laws' house, if I potty him in their sink, I am always sure to close the door because I would feel guilty if someone saw. Perhaps that means he's too old. On the other hand, think I felt the same way when he was much younger.
The change I'm dealing with now, is that when I take him to the potty in the sink, he just wants to play with the toothbrushes, with the thing-a-ma-jig that you push or pull to get the drain to open up, and anything else within reach. It drives me nuts! If he REALLY has to pee, he will. But if he can still hold it, he'll play instead

We have also been holding him over the toilet also, since very early on. We have a stool we keep in the bathroom, and we sit on that facing the back of the toilet. But these days he expects me to start out at the sink and gets upset if I start out at the toilet (DH can take him to the toilet, no problem). If he's at the sink and begins to poop, that's when I make the shift, and he's okay with that. I'm not really happy with the situation! I recommend you switch before it becomes a problem!
We also have a couple of BBLPs but haven't been using them much lately. Now that DS is mobile, I think I'd like to start taking him to the BBLP more often, so we can keep it nearby, and he can go to it to let me know he needs to go.
Sorry to stray from the crux of your questions. But the sink issues have been on my mind!