Repair table field: Import/append records from specific backup field if they are not null
There are two solutions I can immediately think of for your current situation:


Write a tool to parse the MySQL dump file, extract the records you're interested in, and insert them.
This sounds like a lot of work, and a lot of time (especially if your dump is large).
Do as you suggest in the question: Reimport the data somewhere and use a SQL query to get the fields you want.


My vote would be for (2) because you can be more precise about the selection of the rows (and the SQL will probably be faster than writing a script to parse the dump file. You'll need a machine running MySQL, with enough room to import your data, but you might be able to do that on your main server by restoring to an alternate database.