If one is ready to sacrifice the possibility of adding favorites directly from the Boom front panel, it is easy to map up the preset buttons to support more than six favorites:

preset_1.hold                           = playFavorite_7
preset_2.hold                           = playFavorite_8
preset_3.hold                           = playFavorite_9
preset_4.hold                           = playFavorite_0

However, since there's only room for 10 numbered favorites, one can't map up all six buttons to a total of 12 favorites.

If Squeezecenter supported 12 numbered favorites, this would be possible.