I think I will eventually add a special page on this blog for slackline festivals (and flow arts festivals). It seems as though the information is very spread out.
The following note by Thomas Buckingham has some of them listed:
#myslacklife SHARE from @burds_eye_view – Manoloco walking my new line "Water Walker" during the #highline festival this past weekend. This was by far the biggest rigging project of my life. It involved 5 ropes, aid climbing and the installation of 16 bolts. Psyched to get the send and contribute to the highlining here in #PotreroChico. Big thanks to @slack_tech for helping me out with the missing pieces and Joel Guadarrama for donating some beefy anchor bolts! @elbuhocafemx