Meadow Park is the only area in SLO where you can legally setup a Slackline between trees. Only certain trees are designated for this in the park, denoted by webbing tied around the trees in a high branch.
As of Late 2014, Slacklining is allowed throughout SLO Parks. Excluding Mini Parks and the Mission. There is current municiple code regulating slacklines and what is needed to be legal.
A Meadow Park Slackline Area rules page can be found here on the city website. Jerry Miszewski, a Cal Poly grad and owner of slackline gear company Balance Community, was instrumental in getting the city to allow slacklining and creating an area for it.
Slackline Tree Location and Distance Data
A to-scale map of the trees in the area and tables for distances between the allowed slacklining trees can be found in this pdf: Meadow Park Slacklining Area tree map and distance tables.pdf