and wrt the other comment, it is totally accurate - changing developer behavior for these search products is tough! we welcome any ideas :)
Bloop looks awesome too, don't get me wrong, and I'll check it out.
I'd say the sweet spot is somewhere between just leaving the competitor's launch thread alone, or, if you must, then (1) mention it once and stop there; and (2) when you have any relationship with the alternative thing, disclose it.
That's the sort of thinking we apply in practice but wouldn't make a formal rule out of, partly because it's always evolving, but mainly because we don't want the list of rules to be too long. If we tried to codify all such things we'd end up with a bureaucratic list of hundreds of rules—ugh!