I'm sorry, I totally spaced on that one - if the /parent param has a filter, you have to use /xpathsrc instead of /child.
concat("Insert /parent=/my:myCPR/my:MainPageGroup/my:MeetingInfoSection/my:Meetings/my:Meeting[", count(preceding-sibling::my:Meeting) + 1,"]/my:Clients /xpathsrc=/my:myCPR/my:MainPageGroup/my:MeetingInfoSection/my:Meetings/my:Meeting/my:Clients/my:Client")