Yeah that's right. It's a script in ./debian called postinst or <packagename>.postinst that runs after the installation. It sounds like what you want is indeed a directory in opt, probably a subdir of the app directory so that you can just put the data in there. Then you'd have a line in postinst:

chown user /opt/myappsdir/data

Make sure you create the directory in the install or postinst and that uninstalling the package properly removes it. Make sure that any data files that get installed during package install are also changed to user ownership, so they can be deleted by the app later on.

There's almost certainly a better way to do this. It wouldn't work if e.g. the user had changed their user name to something else. This is just what i've used and it works ok in a single-user N900 setup.