I guess this is impossible to do?

is there any workaround for this case ? 

for example, maybe we can solve this case with change settings the sharepoint alternate access mapping ?

Hi, this is an old post however I also had the same issue using "web URL" in emails sent by a Nintex workflow.

I solved this simply re-ubishing the workflow and using the public website (the same was previously published with intranet web applcation).

I guess that Nintex workflow compile the current "web URL" when publishing the workflow, it doesn.t consider AAM or extended web application, simply the current one.

Hope this help!