I just ran into this problem for the first time today (game#: 81356152). My opponent was kind enough to walk through a test run, and we confirmed that there still seems to be no way to choose WHICH Border Guard to TopDeck when multiple BGs are in play.
This forces any player with both "The Horn" and a Border Guard that is set aside with Prince to choose between losing a "Prince'd" BG and nullifying the effect of "The Horn".
I am replying here because this thread is the most recent one I could find on the topic. The point of my post is, shouldn't this be part of either the Known Card Bugs or Known Interface Bugs threads? I don't see it listed on either one. Just don't want the issue to get lost in the shuffle, so to speak.