Perhaps using “Random FX Value“ step fx would do the job?
f - Random Fx value, sets the range of randomization for the Fx that is in the same step with each sequencer pass. For example, if the randomizer value is 10 and the affected Fx value is 100, the algorithm will pick the value from 90 to 110 from +/- 0 to +/-255.
This brings an idea… I haven’t tested this, but… Slide up/down values go from 0 to 255. You can’t select negative values manually. However, with Random FX you could set a range that (at least in theory, mathematically) could bring Slide Up to a negative value. And if that can happen indeed, is a negative Slide Up value applied as a Slide Down?