Can you post a link to the registry hack you tried? The steam distribute of the game comes with its own copy of java_vm, so you have to make sure registry changes are applied globally, or to that the specific location since its not using the system default.

But part of the difficulty is that starting the game is a 2-stage process. Stage 1 launches getdown to check for patches/updates, then initiates Stage 2 where it launches the game under a separate process before terminating itself. Any non-global parameters applied at launch are subsequently ignored when the game launches (the one you want modified) because its not setup to pass along additional parameters. You also can't modify the file used to drive the game's launch options since its monitored by the patcher.

This leaves you only 1 workable option. You can replace javaw.exe with a bat script with the launch options you want and convert it into an exe. Crafting a regular shortcut won't work due to the length of the of the classpath options, and the fact you can't get Steam to use it anyway. The link below shows instructionson how build a bat script to launch the game using custom options, and you'll then need to convert it into an exe and name it javaw.exe.

The catch however is the patcher won't run in this setup, so you'd have to put back the original javaw.exe in order to get updates.

http://forums.steampowered.com/forums/showpost.php?p=24184457&postcount=10
http://forums.steampowered.com/forums/showpost.php?p=24291569&postcount=20

And if your thinking of making a custom shortcut and loading it through Steam as an external game, it doesn't work since the client won't be able to detect the Steam client running when you do this..... which is still a mystery as to why.