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:
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.