Without boring you with the details of "why" to keep a long story short, I need to change the domain that my site uses when it sends pings. I went and searched the WP source for where this gets set, and I see the domain is set in weblog_ping() in wp-includes/comment.php. Unfortunately this has no filters or actions, so no dice. It does use home_url() which ultimately has a filter, but I only want to filter it when it's being called for use in a ping. Anyone have any thoughts on how I might accomplish that?
... And as I was about to hit post, I had an idea of how I might do this. I'll post it as an answer, but I'd still love to hear what others have to suggest.