In reply to:
Baloney. I've done it a couple times now.
Well actually, you glossed over a number of important points. As you say, you need a datasheet, which means no buying a cheap broken laptop off ebay or similar. You also need to be confident wiring the thing up - which depends on the soldering skills of the OP.
Also, you need to source all the parts, which as you mention is not exactly simple for the occasional amateur electronics enthusiast.
Also, the OP wanted the system to work in Linux - I don't know, but I might imagine that the majority of cheap controller boards for running LCDs would not be well supported in Linux.
None of this is unsurpassable, but it is a
lot of hard work, and if you don't know a fair bit about electronics, have the ability to source components, and possibly have the knowledge to write your own graphics card driver for Linux, might as well be close to impossible.
A much better solution is the one the OP suggested, which was to use a desktop LCD and remove it from the case - it can then be easily powered, uses a standard 15 pin VGA cable, and can be driven from any graphics card you like.