How do I vary PathAuto paths based on a field? I have a field type called "Venue". If the Venue is in the US, I want the path to be:

/CITY/my-venue


But if it's in any other country, the path must be:

/AU/CITY/my-venue


In other words, non-US venues must get their country code appended to the front.

Is this even possible with the current PathAuto module? Or how can one do this?
The easiest approach is to define a custom token using hook_token_info() and hook_tokens(). Based on the venue, you either return the country code or nothing. If the token has no value, pathauto will automatically clean it up and drop that part.

Pathauto tokens for taxonomy terms except root for example has some hints on how to implement custom tokens.