When you defined the Custom Action, did you create an Exe types Custom Action that points to the Quicktime Exe? If this is the case it won't work because the file is streamed in to the setup and extracted to some temp location during the install (minus the qdat file).
Couple of Suggestions.
1. Use Package For The Web (or some other similar product) to wrap the Quicktime Exe and the qdat in to one self extracting Setup.exe. Call this exe as a Custom Action. This will then extract everything to a temp location which includes the qdat file as well.
2. Leave the files in "Support Files" and write a Custom Action (in C++ or VBScript) that run's the exe located in the Temp Folder.