The salt would not be the main problem, the chlorine in the water would be an issue though
If you can let your FC drop to low levels and ph is at normal levels then it should be fine for the lawn, as grass is fairly salt tolerant. Some flowers and shrubs are susceptible to salt though, so I would be cautious about disposing to a flower bed
If I had to drain to the garden (and I do when I backwash the filter) I do it at low FC, less than 2 ppm (the lower the better), and do so before rains, so the rain dilutes the salt water. I also try to drain it to different places to prevent build up of salts and boron in the soil.
The other thing to check is are you allowed to drain to the stormwater system, I am not allowed to do that here. Although I have no idea how they would find out it was me ;-)