135.379 is totally relevant to tracer997's scenario. And nothing was said about losing an engine, which is off-subject. As I read it, the question is simply...
"If I am too heavy to meet the climb gradient on the ODP, can I still take-off at the heavier weight and maneuver visually to avoid obstacles under Part 135... where is the reg??"
135.379 states obstacle clearance requirents both within and outside of the airport boundary. And AC 120-91 reminds us that "operators MUST COMPLY with 14 CFR requirements regarding the development of takeoff performance data and procedures." That is, 135.379.