Can Mac App Store purchased apps install Quicklook Plugins?
Yes, any application placed in the Applications folder can provide QuickLook support.

Sandboxing does not affect an application's ability to support QuickLook.

Mac OS X looks inside applications for a QuickLook plug-ins folder. The specific folder is:

MyApp.app/Contents/Library/QuickLook/


Support for Automator and Spotlight can also be provided using this per-application Library folder mechanism.

When you download an application from the Mac App Store you are still going through /usr/bin/installer. The Mac App Store downloads a flat-file installer package .pkg and then installs it on your behalf.

This allows the installation process to be more than just a single copy for complex applications; functionality that is only available to Apple software. For now, third party software can do no more than provide a single application to be placed within the /Applications folder.