There is a lot to be said for people rather than machines, it's difficult certainly to design a vision system which is as good as a "fresh" person. That said, how "fresh" are your people? Are you rotating people on jobs? People become blind to things they see hour after hour or day after day. Also do you define your standards well on what is acceptable vs. unacceptable? They must, must, must be visual.
I would say it would be difficult to have as a CCP and if you think it should be a CCP, I would still invest in detection equipment but if it's not a CCP in your process, then perhaps it's an option; after all what is glass control but minimising the risk then visual inspection and awareness?
Perhaps the best place to start thinking is "what would happen if it went wrong?" because people are fallible but are not permitted in law to be(!) So if a machine broke but you did everything within your power to keep it running causing a food safety issue, then that *might* be ok but if a person missed something I think a court would expect you could have reasonably predicted that could happen, therefore would come down with the full force of the law; especially if there is a (perceived) foolproof machine which could have done the job.
So it does depend on risk.
Personally I hate magnets though, they have a nasty habit of dumping the contaminants back in.