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I've used the Canon 135mm f/2 for FD mount without any issues. I've also used the FD 200mm f/2.8, and the mount seems just fine, though I haven't shot tripod mounted with that lens off the end. However, I don't have any issues with the lens hanging off the camera. I do support the weight by the lens when shooting, though.
I wouldn't feel safe with a longer, heavier lens, though, as the m4/3 mount flanges are just pretty small...I'd be worried about the bayonet flanges on the lens adapter, which could possibly cause the lens to come crashing to the ground.