Reviving this topic after 2.1.0 update:
I have been keeping a Boss DD-500 on my board along with my QC for other delay types. Reverse delay was one of those, but the QC finally got it with 2.1.0.
DD-500 also has a mode called Slow Attack which I use a lot. This mode allows you to choose a volume threshold, rise time, fall time, etc.
If this auto swell function can get implemented in the QC, I think I may not need the DD-500 anymore. It should have the option to toggle rise/fall times with tap tempo, or have fixed rates.