It would just be hard water, so IMO you could do whatever you wanted with it.
If your RO reject waste water is say 4 gallons waste to 1 gallon of good RO water - then the ratio would be 4:1
The mineral content in the waste water would roughly be 20-25% greater per gallon ( very rough estimate ) than your normal tapwater.
Watering plants or garden would be okay I suppose.
Or washing clothes, car or other cleaning uses.
Realizing that the higher minerals may leave water spots on the car or require you use more detergent/soap for laundry or bathing.
If you have a water softener on your tapwater, that feeds the RO unit, some of the minerals ( calcium/magnesium ) would already be reduced.
I probably wouldn't make a habit of drinking it, but that's more from a potential bacterial reason than anything else. The surface of the RO membrane could house bacteria in non chlorinated tapwater, such as in wellwater.