I have two overflow chambers, and I am interested to know if there is an issue with placing a skimmer inside one of the chambers.
It will need to be a relatively small footprint, possibly a Tunze 9005.
What I did was add a 5 gallon bucket for my protein skimmer to sit in. My overflow dumped into the bucket then I plumbed the bucket into the sump. I used a hole saw to drill the holes. Then added the uniseals. The pvc pipe is then inserted into the uniseals creating a tight fit. All this fits underneath my 55 gallon tank.
I don't see a problem with doing that. Its basically the same scenario that occurs with nano tanks and protein skimmers in the chambers. Good luck. Maybe someone else can chime in with experience.