Andy, at least on OC I never had to add extra weight with any of the FFMs I tried.
Pip, most likely the connectors for the comms would come from the comm manufacturer. As I mentioned, OTS has a P-plug for their comms, the Buddy Phone as well as the larger units (2010 etc). The Buddy Phone looks like the GSM, but is hardwired. The 2010 has mikes and speakers separately from the electronics, which can be mounted anywhere.
Ocean Reef should know if the GSM can be used with the Dräger.
If you get no reply try
Ocean Technology Systems if their mike/button plug will work with your GSM, or if they know of a Dräger/GSM setup.
OTS used to make comms for the Ocean Reef mask, and they do make them for most other FFMs, so I'm quite sure they'll know. John Hott is good person to contact, info on their website.