Just a thought, but could the Cabin Crew not be used to "Observe" on engine startup for any abnormal smoke/flames etc... on engine start up? Just a simple addition to procedures to have one standing by with a view of the back of the engines Flight Crew makes some kind of announcement "Cabin Crew standby for engine start", a crew member walks to a position where they can see the engines through the windows with another crew member upfront positioned with access to the intercom who can relay any signals from the observing CC a second call "Engine start complete" from the Flight Crew allows CC to resume normal duties.
Obviously for wide body aircraft one down each aisle perhaps required?