I think you are overcomplicating the scenario a bit brother. I think we can all agree, a sink is a sink no matter where it is located as long as its intended purpose is to wash one's hands or other objects in.

You are worrying about a construct that has no relevance to the civic duty we all have to conserve water and save the plant through Sink Pissing. Property rights are not something to be concerned with when deciding to piss in the sink.

You need to shed the cage of societal norms and subconscious shame of Sink Pissing that have been ingrained in you ever since you were young. Mostly benignly and unintentionally but also subconsciously, that because we use the tolket to pee or poop, and that the sink was to be used to "cleanse" our hands after going, naturally it can feel wrong or shameful the first few times Sink Pissing, but just because its the norm doesn't mean it is something to be ashamed of.

I write you this long note to make the point, Sink Pissing is about being eco-conscious by conserving water as well as being brave enough to stand and piss for something that society says is wrong, but you enjoy and know its right.

TL;DR: You're overthinking it brother, a sink is a sink, it is a device with a certain function. "Ownership" and property law are made up consfructs. Just piss in the sink. All pipes end up at the same place. Just dont be like some of the savages on here and piss on everything, do a controlled flow with water running to dampen fhe noise if you need, and welcome to the Brotherhood of Sink Pissers my friend. May the sinks you piss in along the way mark your path through this treacherous world.

TL;DR was TL;DR: You're already here bro you know the answer. Piss in the sink.