You need to define what you mean by "no sterile cockpit". Are your CC calling you during the T/O roll to chat? Do they walk into the cockpit at 5000ft in busy terminal airspace to "see how you guys are doing"? Or does your company define sterile cockpit to mean "you must never contact the crew no matter whay"
Like Check Airman said, sterile or not any FA should if he or she suspects a fire, call the cockpit immediately. Sterile just means no disturbances EXCEPT for emergencies. Having the Crew Intercom turned on and with volume up simply ensures that if you do here the buzzer during T/O there is a step you can skip, if anything this isn't such a bad thing. My company insists on a sterile cockpit, but if the PM heard the buzzer I'm sure he would answer the call.
The reality is just go with it. If you're passing 100kts and you suddenly hear someone screaming "FIRE" in your ear, reject the take-off, and let the company deal with it. They are the ones that insisted on this procedure. They must trust their CC a lot and believe that at the end of the day having this open line of communication is a good thing. If it turns out that it was nothing more than dry ice then so be it, its the companies problem, not yours.
Just on a side note, how experienced are the crew in your company? Especially the guys and girls in the back. Whats the average age of the most senior of the guy/girl in the back.