[TxMt] defaults OakShellVariables

I'm looking for an easy way to add a shell variable to TextMates preferences via a shell script, I've got so far but the final step seems a little illusive.

The following just overwrites what's already there....

defaults write com.macromates.textmate OakShellVariables '{enabled = 1; value = "/Users/simon/classes"; variable = "CLASS_PATH"; }'

As the script would run as part of a bundle package install it would be possible to quit textmate first and not run the risk of defaults confusion, but ideally I'd like something similar that could simply be executed via a command.

Any help would be great.

Thanks, Simon