How to create a OS X installer for an app that contains a plugin?

My app is properly code signed...

codesign -vv MyApp.app
MyApp.app: valid on disk
MyApp.app: satisfies its Designated Requirement

spctl -vv --assess --type execute MyApp.app
MyApp.app: accepted
source=Developer ID
origin=Developer ID Application: Me / My Company

It contains a plugin at MyApp.app/Contents/Plugins/jre.

If I create an installer...

productbuild --sign 'Developer ID Installer: Me' --root MyApp.app 
     /Applications/MyApp.app MyAppInstaller.pkg

... then install it, then the installed MyApp is no longer properly code signed.

codesign -vv /Applications/MyApp.app
/Applications/MyApp.app: code object is not signed at all
In subcomponent: /Applications/MyApp.app/Contents/PlugIns/jre/Contents/_CodeSignature/CodeResources

Other notes:

• The installer itself is code signed, just not the installed application.
• I get a properly code signed installed application, if I do not include the plugin.
• I get the same result if I use PackageMaker or productbuild.
• I don't care about the App Store. I just want a properly code signed installed application.

I tried usingpkgbuild to build a separate package for both for MyApp.app and the plugin, then combine them using productbuild, but I got the same result. I am unsure if the pkgbuild step is even necessary.

What am I missing?

This appeared to work for me:

productbuild --component ./MyApp.app /Applications --sign 'Developer ID Installer: Me' MyAppInstaller.pkg

I guess it was necessary to use --component instead of --root.