Turning off built-in monitor during boot from Trisquel 7 USB

Last night I was helping my buddy fix his laptop. He's been running Ubuntu for a few years, and I was trying to convince him to try Trisquel. The built-in screen on his laptop is cracked and completely unusable, and he uses a Panasonic TV as his monitor, connected via an HDMI cable.

I was able to boot up from a Trisquel 7 boot USB, and the initial menu displayed on the TV, but when we actually booted into the live OS, it wouldn't display on the TV and we were stuck. We ended up reinstalling Ubuntu 17.10, because its live USB displayed on both screens by default :-(

So my first question is, is there a way to boot into a live Trisquel 7 and get it to turn off the built-in monitor, and use a screen connected via HDMI?

My second question is, can the Trisquel 8 live OS be configured to making 'mirror display' the default behaviour when there is an external screen plugged in, or at least give the user some display options in the live OS boot menu?