I couldn't find this documented anywhere, but the option exists and lives in the code!
If you're using python, selenium, and phantomjs, somewhere in your code you have a line like this:
from selenium import webdriver
driver = webdriver.PhantomJS()
To enable remote debugging you need to know that PhantomJS() takes a 'service_args' argument. Change the second line as follows and you should be good to go.
driver = webdriver.PhantomJS(service_args=["--remote-debugger-port=9000"])
You can then follow along with the various tutorials on remote debugging for PhantomJS. I like this one:
https://drupalize.me/blog/201410/using-remote-debugger-casperjs-and-phantomjs
To learn about more hidden options for PhatomJS, read the code!