Holy crap, I never knew about this before, but this is awesome.
The command is drush uli
It's got options,
name
for the usernameuri
to specify the url of the site (our weird setup requires us to specify the url of our site)
ex: drush uli --name=rootid --uri=http://midpen-openspace-d8.test
A window popped up that led straight to the user page, with its "reset password" fields! I'm using it on my local, so I'll need to redo if I pull the database from Pantheon, but I'm only doing this cause Jason's out of town so there aren't any changes to pull anyhow!
terminus drush <site_machine_name>.<env> -- uli --name=<username> --uri=https://dev-midpen-openspace-d8.pantheonsite.io
Note that a window doesn't pop open, but after a warning about the site being in Git mode and possibly adding the site to known hosts, the login url was output into the terminal!