I have tried to perform this in my test env and... this didn't work,
my conclustion is : common_name_attr didn't work where common name is derived attribute (or didn't work with cnt factory due to some scripts).
So you need to refer to plan B,
- you can create new table, where you keep reference between contact (SREL to cnt table) and his Client ID, display name will be Client ID field;
- form input will be dtlLookup to that new table;
- if I have the same Issue I'll keep both fields (Client ID input and Requestor name input) with some synchronization between them;