I wonder how many players will each side have. Also im sure many guys plan to make beastment and after completing all the ability quests on Arboreus (even the "beastmen only" ones), they´ll convert to abomination to collect all the Tartarus quests and go pvp (because as devs said, beastmen are the worst choice for pvp, so they have no other possibility).
Can´t beastmen really participate during invasions? In that case they´re not lawfull/neutral good, but rather true neutral and it´s pretty sad they can´t choose to defend their fellow "goog guys" in fight, only attack them as abominations.
The simplest solution (in case i wanted to help humans) would be to turn to abomination and upon start of "invasion time" teleport to Syndesia and start hunting demons. Little overcomplicated, but would this possibility work ?