Thanks to @vinothkannans’s primer above, I managed to put together the discourse-TL1-invites plugin. 
My first one, yeay! 
And the best thing is: it works (from what I can tell so far).
The only thing I am a bit unsure about is that I am not 100% sure whether I am allowing TL1 users anything apart from forum invites and topic invites here:
https://github.com/tophee/discourse-TL1-invites/blob/master/plugin.rb#L23-L38
The only change I made is obviously the trustlevel in line 37, but I don’t quite grasp, what all the possible objects are that TL1 users can do invites for…