So, a dummy hdmi plug will do the job?

No need for dummy monitors, to disable the forced automatic sleeping behaviour when closing the lid just run

    sudo pmset -a disablesleep 1

I remembered that you used to need a kernel extension for this because the behavior is hardcoded without an option. Glad that they added a proper option so you no longer need to do this.

Dummy might not do it. You could need a full fake display adapter - cutting the end off an HDMI won't do the bidirectional stuff monitors do these days, I believe.

A dummy plug isn't just the end cut off of an HDMI cable, it actually simulates a monitor with EDID.

Amphetamine.