I'm sorry i don't quite follow. How is your example using the map directive? Just for clarity, here is what I'm trying to do.

location /[resource]/v2/ {
proxy_pass http://app.domain.com/api/[resource]/v2;
}

In fact, the location bit will be hardcoded with the actual resource, I just need the proxy_pass to grab that value and pop it into its path. So more something like this:

location /users/v2/ {
proxy_pass http://app.domain.com/api/{{users}}/v2;
}