This might be too little, too late, but I perused my 1999 UL Electrical Construction Equipment Directory and found a statement that might pertain to your query. On page 27 it states that: "Some breakers are not provided with a means to prevent their installation in Class CTL assemblies. These circuit breakers are for use in old style, non-Class CTL equipment and are marked 'For Replacement Use Only, not CTL Assemblies.'"
So, it might be ok to have the mini breakers if they are so marked. But,if the wafer breakers are modified in order to fit the panel bus bars, then they were illegal from the get-go and you are not permitted to add more breakers to the non-code compliant panel.
Instead of a 200 A service change, it might be feasible to do a panel change to allow more breakers..
