So I figured out how to get the 1.6 java to show up in the Java Preferences in Leopard. While this does let other apps "see" the 1.6 java. There are still other issues that prevent it from being used in the few apps I've tried. (Jdownloader, phpstorm 2.1.5). Maybe someone can do the leg work to try to figure out the remaining parts.
TL;DR Version
Download the plist file I attached. Replace the one in /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Resources/Info.plist (Be sure to save the old plist if you want to revert it)
And as I said, there are still other incompatibilities to getting this running with full native integration. But it's at least one step closer.
Full Version
I wanted to also document my observations and possible routes for maybe getting this working.
So the Java Preferences.app itself isn't responsible for enumerating the available installations. It's actually offloaded to another Framework component /System/Library/PrivateFrameworks/JavaLaunching.framework. This reads each folder in the JavaVM Versions folder and reads the Info.plists.
So open the /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Resources/Info.plist and replace the contents with the one I uploaded. Basically we're copying in a bunch of metadata from the official 1.5 version. This will let it be "seen" by the Java Preferences. But of course this isn't sufficient. If one runs a Java application, it won't launch.
A closer look at Console.app shows the java application is complaining about missing dylib files when trying to run an application. Specifically libserver.dylib. It's expecting a specific path for these files. While those files are present, they're in a different directory in the 1.6 tree. Symlinking these files from their original spot to the expected spot lets the application progress further. But we run into another issue.
If we look again in the Console.app, we see the application is complaining about a missing symbol in one of the java library files. _CTFontGetGraphicsFont. From my (admittedly limited) experience, this means the files have been compiled for a different target than the one its currently running on. It's expecting methods and calls that aren't where it expects them to be or not available at all. If the origins of the 1.6 developer preview is Tiger, this would make sense.
The file where the offending call was happening from is the libawt.jnilib. Looking at the decompilation of this file in the 1.5 and 1.6 versions we see the 1.5 makes no reference to CTFontGetGraphics at all whereas the 1.6 version does. And the 1.5 version tries to call something named CTFontCopyGraphicsFont instead which isn't present in 1.6. They're both trying to find this call in the /System/Library/Frameworks/ApplicationServices.framework. I'm guessing this is due to underlying API changes moving from Tiger to Leopard. Even if this call were to be patched with the Leopard version (and it doesn't appear to be a drop in replacement). There would probably be dozens if not hundreds of other such calls to be patched. In short, a monumental task that's above my pay grade 😅. Maybe someone else here has the skills and time to attempt such a task.
What about 1.7?
The uploaded 1.7 installation directory is very different from the 1.6 version. But maybe someone can try to rearrange the folders and create an Info.plist that will satisfy the Java Preferences? I haven't tried it, but it's worth looking into if someone has the time.
Oh and one more thing. -- If you do this and the Java Preferences won't open, it's because its' trying to use the 1.6 JVM which doesn't fully work. Reverting the 1.6 plist to stock will get it to open again.