Please do not deploy this unless you do it in a way that does not leave a trace.
In other words, if you do this in such a way that people who don't care about this can't even tell that it's going on, that's fine. If you have to interact with the HN interface, e.g., by leaving comments, that will become incredibly annoying.
How could you do it without leaving comments? Easy. People who care enough could leave their public BTC address in their profile and install a browser extension that allows for your tip trigger mechanism to be something other than comments. The browser extension could indicate which users can or cannot receive a tip based on a cached scrape of their profile indicating whether or not they have a public BTC address.
In other words, if you do this in such a way that people who don't care about this can't even tell that it's going on, that's fine. If you have to interact with the HN interface, e.g., by leaving comments, that will become incredibly annoying.
How could you do it without leaving comments? Easy. People who care enough could leave their public BTC address in their profile and install a browser extension that allows for your tip trigger mechanism to be something other than comments. The browser extension could indicate which users can or cannot receive a tip based on a cached scrape of their profile indicating whether or not they have a public BTC address.