Hello,
while the FlexResponse allows you to integrate with other applications AFTER a file has been scanned you might want to look another tool or API.
If you need a file checked by DLP i would suggest using the ICAP functionality of DLP. So you will need to set up DLP Network Prevent for Web and this will give you the option to run a DLP scanner as an ICAP server. The application can then send any file to the ICAP server interface over the network and will receive a status code if the file contents triggered any policy.
ICAP is the same protocol that is used by virus scanners to scan e.g. web requests.
More detail about ICAP can be found here:
http://tools.ietf.org/html/rfc3507
A proof of concept ICAP client in C# that works with Symantec DLP can be found here:
http://code.google.com/p/simple-icap-client-csharp/
This project is open source and you can modify the code according to your needs.