Eric,
Here is a method I used in the past, I don't know that it is the best method, but it worked in my case. Hopefully it will help you out or give you some ideas:
Add new text field to All Participating Feature Classes & Their Attachment Tables
Add and Calculate New Field to Equal GlobalID for Feature Classes, and REL_GlobalID for Tables
Merge Feature Classes. Merge Tables.
Add New Long Integer Field to Merged Table (Like REL_OID).
Join Merged Table to Merged Feature Class Based on Text Equivalent to GlobalID.
While Joined, Populate New Field with OID from Merged Feature Class.
Feature Class to Feature Class - Remove GlobalID in Field Map.
Enable Attachments on Merged Feature Class (Creates New Table & Relationship).
Right Click Newly Created Attachment Table and Load Data from the Merged Table.
In the Field Map, Specify the New Field Based on the Field Containing the OID Obtained from Merged Feature Class, Leave the Remainder Default.
Check Results in ArcMap.
Paul