The Blackmagic Television Studio HD provide a talkback option but via an aviation standard jack which is .206″ rather than the 1/4″ jack used for headphones.
I have 4 or 5 pin XLR headsets so ordered ths adapter from eBay (China).
Update: It arrived. quality is good but headset jack is mono only. From the picture it is hard to tell which is the smaller (microphone) jack which I thought was the one on the right.
When using a Clearcom/RTS type headset, the microphone level is to low. I have built a circuit based on http://darrenyates.com.au/electronics/archives/16 with an XLR 5pin (for the adapter above) and 4 pin socket (for the headset).
Warning: Run off 9v battery, or if you want to run a dirty plug pack, use a 317 regulator with good filtering, otherwise you get lots of hummmmmmm.