Have crash investigators ever used pilot cameras to help find the cause of a GA crash?
Absolutely: Here's one of many examples, and you can search the NTSB database for "GoPro" and find plenty more.

The NTSB will use anything and everything available to them in their investigation, though release of certain materials is prohibited by law (e.g. CVR contents are not released, only transcripts).
I believe the NTSB is observing a similar protocol for onboard video: A synopsis is released (including a transcript of conversations) when requested, but the actual video is kept confidential by the agency -- I am not certain in that though, and the NTSB press office could provide a more detailed answer on their rules surrounding cockpit video / audio in GA crashes.