If they are CS apps, you can use BridgeTalk.
No, It isn't. Are there another way to establish communicate between InDesign and third part app?
If I were you, I would use APID Toolkit's subjectModified event to monitor changes to the text frame, and to interact with 3rd party app I'd write a script in platform specific language: AppleScript for Mac or Visual Basic for Windows. Then when the event occurs, it could trigger the script via doScript method.
APID Toolkit supports all CS versions of InDesign: CS, CS2, CS3 and CS4.