Here's how I used to do this when working on a smaller scale project and then I'll write thoughts on how it could be scaled.
These were the steps taken in the process of developing the GUI:
Have the general concept (programmer not involved, the designer decides this with UI/UX or the customer).
Designer and programmer review the general scheme that was created during the previous step and the programmer creates necessary definitions for symbols and other assets that must be present in the SWC that the designer produces as the result of her work.
The designer receives the "dummy" definitions, connects them to the project. Whenever she creates a new symbol or imports an asset, she links that asset to an existing definition.
After a stage, when some of the symbols are already in the SWC, the designer hands the SWC to the programmer. The programmer uses his own definitions (from which the designer has derived hers). The programmer used those definition to write the functional parts of the interface and to connect them to other parts of the program.
By cycling through these steps we were able to gradually add more components to the project and develop parts of it independently of one another. (It is possible to split the workload between designers such that each one will produce their own SWC with the set of definition they previously choose to work on).
The pitfalls:
Designers need to be trained to use the forklfow (very few know how to connect symbols to existing definitions, how to compile SWC). But the functionality is there in Flash CS.
Human errors (if a designer makes a typo when linking - she won't notice it until the library is handed to the programmer).
Avoiding human errors
In my smaller scale project I was just running the SWF extracted from SWC through the swfdump utility and piping it to grep to see if the symbol definitions are present. Given there aren't many, it was possible to do it by hand.
However, since the project you describe is larger, I'd go for writing a script that takes for the input the created outline (ActionScript source file) and the SWC library. Decompresses the SWC, extracts, SWF from it, runs it through swfdump and verifies that all definitions from source file exist in the SWF.
The designer would be responsible for running this script before sending the SWC and ensuring that at least those symbols / assets she has added during the last update indeed made it into the library.
There are still several technical problems with the analysis: duplicated names, possible high level of "noise", it would still rely on a human to run this check instead of making it analogous to commit hook for example. But none of these seems unsolvable - given enough effort and inventiveness it must be possible to come up with an application that'd ensure integrity.
