Thanks for your comments. After some consideration and verification, I came to the conclusion there seems to be no solution at this time, unfortunately.
I tried various approaches using several 'collectible API'. We have IsCollectibleUsable() and UsCollectibleBlocked() for example. But these are for determine if your favorite mount can be activated. In other words, it is currently possible to select your favorite mount even in a building, and these API always return true in that case. That's a bit disappointing for me.
