Has anyone had any experience with senior academics on panels for journals, workshops, or conferences stealing ideas from papers (accepted or rejected) for their own publications

not happened to me personally, but have seen it happening. on one occasion the stealing senior academic got told off (inofficially) by a big name (ecr was protege of big name). most of the time you're probably stuffed. the other one to watch out for: them stealing ideas from ecr's funding proposals they rejected. let us know if you find a good solution.

Not exactly the same situation, but I know someone whose PhD supervisor pipped her to the post and published her PhD research under his name only!

I know a leading academic in my field who does that. One time he was invited by a well-known industry (from EU) for possible collaboration. The firm foolishly trusted him and gave him full access to their "secret plans". Collaboration talks reached advanced stages, but the man chickened out at the last minute. He then used the ideas stolen from this industry and sold them to a rival Asia-based industry. Up to now I just can't understand why someone of his standing, who has won multiple awards would sink so low to involve himself in cheap unprofessional conduct.

Seems like there are a lot of scoundrels out there! I've had some great collaborations with people who are fair-minded, but there are lots of people out there ready to cheat you out of the rewards of your ideas and efforts. Think the point about research proposals is a really good one. Also think that it's also the case that the only good protection is being the protege of a big "name".
Depressing.