Step back a bit.
You can access or set vessel parameters listed on Orbiter's API documentation.
Orbiter simulates the "engine system" and for that yes, you can set fuel to 0. The same goes for radios, for example.
But there's no support for user defined systems, such as your HUDsystem.
You would have to program such a system, and handle it's integration with Orbiter, save and load states from scenarios, etc.
I's not impossible, but depends on what you want to do.
On the previous versions of Orbiter LUA disregarded keyboard events, so it's not really that useful for interaction. Perhaps that bug has been solved, I don't know.