Imagine that the boy's dick could grow arbitrarily long, retaining both the flexibility of a softie and the penetrating power of a hardon. He can also control it about as precisely as a monkey's tail. With me so far? So the boy and girl concoct a plan to stroll around in public while sexing stealthily. All they have to do is wear loose clothing and hold hands. Eh? What do you think? I feel like you could do a handful of fun things with this.

Might be the kind of topic that goes here.
gelbooru.com/index.php?page=forum&s=view&id=2825
Maybe.

Fucking through a wall and stealth sex are common, but the clocest I've seen to a supernatural use of a dick in this way would probably be Okina's "Sei So Tsui Dan Sha". But its a detachable dick, not a tentacle dick exactly. Things like portals and Yukari gaps would probably work well for that too, but it is not something artists utilize as often as you'd expect; that is legitimately shocking.