With locked down I mean if you don't have root access on your own device. Since it's running Linux, you should have root access and thus be able to control everything. So you should be able to use a mini hdmi adapter and use xrandr or other programs to output something on your external monitor.

However, I never used Umbrel so don't know how they setup the system. Not entirely sure how it will work but I am 100% sure there is a way.