Hey,
I managed to do them (Special Event is seamless, while Hero Selection required more editing and there is a point with the background that limits me from perfectly looping it - I also tried copying other instances of the audio clip to make it flawless, but I think it works fine).
The downside is that they have to be *.wav, because .mp3 tends to auto-create a small gap in the beginning and/or the end of the track, thus making it sound almost like the original (actually it sounds like a sound glitch, whereas the original sounds as if it's not destined to loop). Why is that bad? Because it takes up much more space.
Here is a test map; quite frankly, I wouldn't really catch the clipping of Hero Selection track as a player, unless you brought it to my attention.