Did you read anything I've said in the last two pages

It CAN be done with the official sdk if you have a recent version.
It CAN'T be done with anything other than the sdk AT THIS POINT IN TIME.
I am just starting looking in to doing it directly on the 360 although it likely isn't possible.
Although I haven't personally tested the following, from experienced I've gained looking in to this shit today it should work and show achievements etc in the dash for full games ... IF YOU HAVE A RECENT VERSION OF THE SDK:
Copy game to HDD using the official copy function in NXE.
Copy the game from your Xbox HDD to your PC.
Open Le Fluffie and point it to the PIRS file of the copied game. Then point it to the data files of the copied game.
Extract the contents, it should produce a single large file (It takes all the data out of the data files and restores it to the original xsf image).
You now have the xsf image of the game, using gdf2content you can now build a new games on demand container... Input will be the extract xsf image and the output will be wherever you want to save the new container to.
Copy this new container to your Xbox 360 and (in theory) you now have the file showing up under Xbox 360 games and listing all the achievements etc just like a hdd installed game or retail disc would but without the restriction of needing the disc in the drive.
AS I HAVE SAID SO MANY DAMN TIMES NOW YOU WOULD NEED A RECENT VERSION OF THE SDK TO DO THE ABOVE.
If/when x360.dll is updated with better support for these games on demand files then I can make a nice and simple one step tool to do this all automatically without the need of the sdk.