At a guess I would say that employers will state a set of loyalty and experienced based minimum requirements to be considered for command assessment which will neatly dovetail with seniority, be it official or otherwise, so, "Pilot A" arriving in a company with thousands of hours and years of experience will still have to do five years as an FO and by that time their "seniority" may have them in the promotion zone anyway.
Companies tend to avoid employing people who are classified as 'over-experienced' for the job, but they could always take "Pilot A"on as a direct entry captain as to employ him as an FO would be to discriminate against him compared to the other company captains who have the same or less experience! Unfortunately the FOs can't cry 'discriminatory foul' as they are not on equal terms by virtue of experience with "Pilot A" - indeed their only saviour would be a seniority system!
At the end of the day the companies will promote who they want to and legal action by employees will simply mark them for life in what is still a comparatively small industry.