Several historical notes,
In 1997 Mesaba did take air frames from Express. Mesaba management need the crews to come with them. Mesaba could not staff them. Mesaba limited the number of Express CA's to 30 (out of 250 on the seniority list) and took the rest of the upgrades for themselves. The rest of the pilots that went from Express to Mesaba were FO's that went as newhires. When Express ended its SF3 operation in 2002 more SF3 airframes went from Express to Mesaba. Express made no claims to Mesaba's list at that time - no one wanted to go.
In the 2003 time frame the Mesaba MEC tried to make the case that due to the reduction in flying at Mesaba LOA 21 applied and Mesaba pilots had rights to the Express seniority list. The Mesaba MEC took the claim to the National Executive Council, where it was dismissed. The Express MEC was informed of the claim, it was not 'brought to them'. The PCL MEC had little to do with it.
If any of you want to see how an SLI is done read the recent SLI award in the Repulic/Frontier/MidWest/Lynx merger. Four different carriers, four different unions, equipment from A320's to Q400's, one pilot group with 75% of their pilots furloughed and another with all of their pilots fired, one carrier's lowest paid pilots made more than the other carrier's highest paid pilots. Arguably one of the most difficult SLI's ever done. The Arbitator did an excellent job explaining how he arrived at the award. In the end he varied very little, if any, from the decades of mergers before. Don't expect anything different in the PCL/MSA/CJC SLI.