Their is no such thing as a registered Pnet user, and if he is sharing games with you he certainly is not a developer who sticks to his NDA, a dev would never hand out a game to someone who is going to blab to the internet about it. Plus if he were a real dev he would have no interest in a hacked retail machine as his company would provide him with development kits. When you sign agreements with M$ regarding development secrets, they hold you to it and you can have severe penalties beyond just losing your job if you break them.
It's true that Pnet would probably be safe with a hacked Jtag like that, but you never know, you can still get a kv_dec out of an old devkit by certain methods. And they are easy enough to get now that it wouldn't be a problem for someone who may want a Jtagged but is a developer to just get a devkit, and of course you can't get everyone to listen and stay off of it....but M$ can also brick devkits remotely and were doing it a couple months back. The bottom line is if you don't need a devkit to develop you don't need to own one...if you have it to pirate games just go back to Jtag modded systems.
Now to answer your question...Jtag modded systems will run dev signed code , if you are getting a dirty disc error then it is missing a needed file, if it flat out won't load that means the Xex is crashing before it even gets going. I have an XDK so i can debug and find out what is causing those problems and that is what i have seen.
You could try signing it retail though you would possibly have better luck, but they key point is whether it is a dirty disc error or cannot start game. the latter meaning the xex is failing, the first one meaning it started the Xex but couldn't find something, which could be a missing dev library, or it could be an incorrectly signed DLL. If you have no Dlls then no the Xex is all you would resign. If you just run gamehacktool on the whole game folder after extracting it, it will automatically scan all directories and resign the files it finds that it handles.