Are there any plans to add this?

http://www.wowhead.com/legion-assaults-guide#transmogrification

Either to show all the time or only when an invasion is up (if that is even possible).

These items are added to the addon under the World Events -> Legion Invasions. Currently we have not found a reliable way for the API to tell us "This zone is under invasion" and allow it to return proper map information. One solution we could do is making a Legion Invasions header under each zone as well. It's just a matter of finding a decent enough placement so when it shows up in the minilist it's not always spammed at first (for those who don't care about it), but also visible enough to remember to maybe do the invasion.