Since then I've realized that to indicate that you want an app to use more memory if available or to require that your app must have more memory to run (and therefore can't run on low memory devices) also requires manually editing the WMAppManifest.xml file.
So, as you may have guessed, I've built some nuget packages that can set this up for you.
To indicate that your app wants to use more memory, you can update the manifest to indicate this, search for "WMAppManfest-Extended-Memory" or use:
PM> Install-Package WMAM-FCEM
Alternatively, if you want to indicate that your app should only run on devices with higher memory levels, you can update the manifest to indicate this, search for "WMAppManfest-Higher-Memory" or use:
PM> Install-Package WMAM-RM300
As with my other packges which manipulate the WMAppManifest.xml file, the changes that are made aren't undone when you uninstall so you can (I recommend) uninstall the packages straight after you've installed it.
You can also find these packages by searching for "WMAppManifest" in the package manager.
Read more about memory limits on Windows Phone 8 at http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj681682(v=vs.105).aspx
If you use these packages or have any feedback, I'd love to hear it.