I believe it has to be in the post-installation script, because if you put it in preinst, you'll update the cache before your icon has been installed.

In addition, related to automatically pulling in dependencies, you need to create a Debian source package, because PyPackager made debs cannot do this.

There are different ways to go about doing this, the easiest I found is just using Scratchbox and the debian/dpkg tools (if you don't have Scratchbox, you can use Easy-Debian or similar on the device itself, too).