In the days of System 7 someone wrote a little piece of software called "veraeusser" to do exactly the trick you are asking for: Deactivate the internal display in a PowerBook while driving an external display. Unfortunately I can not locate a copy of this. Probably a fellow comrade will provide it to you. Please be so kind and post if (where) you found it. With best regards.
Many rubber feet cause discolouration of the surface they stand on (like a table with wooden/plastic/laquered surface). You can easily make custom feet from wine cork (natural corc, neither plastics nor composite corc made from ground corc leftover). Just cut it to the desired size and attach it with adhesive tape or some neutral bonding emulsion/glue. Natural cork will usually cause no issues regarding discolouration, as long as you use a glue that will not act in this way. For example, Pattex glue _will_ cause discolouration, diffusing through the porous corc. The cork itself is neutral to most surfaces. It can be cut with a sharp knive, better with a fine saw. Cork can be ground to exact dimensions easily with sandpaper. Just cut out a set of corc feet, grind the feet to the same thickness, attach them, and the Mac will happily stand on environmentally acceptable antiskid feet
Most likely the schematics behind this are simple, as Apple would not have used a different controller to enable this function. Is there a pinout available? Probably one could wire a PB1xx the same way and use it with the driver mentioned above.