A great suggestion

You can't change APPDATA as most apps don't use it. You can change USERPROFILE but that introduces a whole host of other problems. There are a couple commercial solutions that promise to let you make any app portable but they are exceedingly hit and miss and not as stable or reliable as the platform and apps we release. And they don't support apps most people want like Office or Photoshop anyway since those have hardware-tied DRM in them.

To answer that question: there's a reason why the platform doesn't directly make apps portable.
From the beginning, PortableApps has been working to create individual apps that are self-contained and portable. Each app lives entirely inside its own folder, so for example, everything that OpenOffice Portable needs to run is included inside the OpenOfficePortable folder.
This way, you don't NEED to use the PortableApps Platform to run PortableApps. In fact, the Platform in itself is really just a menu of shortcuts to portable applications, with a few bells and whistles added in: it has a very simple backup routine, some shortcuts to subfolders and the Windows search routine, and a routine to check the Sourceforge site for updates to apps hosted here.

What you are suggesting assumes that with a few simple tweaks, any app can be made portable from a central point, but that's not the case. Every application has different dependencies, different registry entries, etc., so each app needs to be individually re-configured to be portable.
Thankfully, one of the developers here created a system (the PortableApps Launcher) that simplifies the process, so that almost every adjustment can be made using an ini file inside the app's folder. Each app still needs to be configured individually, though.