Adding arbitrary code to ArcGIS ModelBuilder?

Is there any way to add arbitrary code snippets to an ArcGIS model?

I envision something similar to the "show code block" the expression builder in advanced labelling or field calculator.

For example my project of the moment could benefit greatly from fetching the username of the person running the tool. I know in python this is a simple import os; os.environ.get('USERNAME'). I think I could write that snippet to get_username.py, add it to a toolbox in Arccatalog, then add that tool to the model and use it as per-requisite model parameter for my current tool. That's a lot of work for such a small thing, and next time I might want something else, like inline variable substitution or a compound variable.

I think a code snippet tool or similar could be address all of these needs in a generic, broadly helpful manner.

Answer

New answer to an old post, just to clarify how to create a re-usable model to do this.

Open the model, Insert-> Model Only Tools-> Calculate Value.

In the Expression, type: getuser()

In the Code Block, type:

def getuser():
    import getpass
    return getpass.getuser()

(See "Getting user name in specific format" for source of username code snippet from @ChadCooper)

For Data Type, choose String.

Rename the output circle to username. The end-result model will look like:

And you can then just drag this model into any future models and reference the variable called %username% in any future models or scripts. Just be sure this model runs before you try to access the %username% variable. You can do this by using a precondition.