I've been giving some thought to this....I don't own a Fermzilla, but I'm trying to figure out a good way to do an O2-free dry hop in my kegmenter and a whole Fermzilla is cheaper than any of the ideas I've had so far.
What bothers me is that even if you purge with CO2 to only have a small amount of O2 in the collection container, when you open the butterfly valve it's going to bubble through the beer to get to the headspace.
Using marbles or similar to reduce the amount of volume in the container should reduce the amount of O2 proportionally, so that would help. So too would filling the container with an O2-free liquid along with the hops. The most obvious choice is the beer itself....with a carbonation cap and a bit of silicone line, it should be possible to do a psuedo-counter-pressure fill of the purged collection container in-place on the Fermzilla without splashing the beer around excessively.
Alternately, you could push in boiled water, but that would require introducing quite a lot of water.
But perhaps that becomes viable if you fill the excess space with marbles.....very little air displaced by very little water should get you a solution where very little O2 is bubbled through the beer when the butterfly valve is opened.