Ideally a sequence would have a "hidden" rack and the little panel at bottom of timeline where you can mute and tweak individual sequences had a button to add rack units directly to the sequence.
Another way that I know should work is to have a sub track with say delay effect on it and if I want to add delay for just a single sequence I can just drop it down to the sub track. I did try to configure this but it got messy. What a would like is a the sub track assigned to a sub rack of the track rack
