Hehe, for sure. I guess I prefer hacks where you can plug some doodad into any old Mac, and it'll work, rather than things that require special software or drivers. Seems more "pure" somehow. As for WHY I'd want to mirror the display, because I can!
I suppose you could hook it up to a KVM like Trash80 suggested, or project it on a big screen to an audience or something. You could even remove the Mac motherboard and run it independently of the rest of the Mac (the analog board, CRT, and case), powered from a 5V wall wart. Make a headless Mac Plus! Put a Plus board in an LC case! Who knows...Yup, I just read a book that described how to do the TTL adapter, which is what got me thinking about this.
That sounds good too! I'm not sure how you'd physically connect to the bus on an older Mac though - some kind of clip-on? What kind of hardware would you use to snoop the bus? I'm not sure the RPi would be up to it.
Now that I think about it, you could combine our ideas, and use a CPLD/FPGA to capture the video signal coming out of the logic board, then pass that to a Rasberry Pi or whatever, instead of driving a VGA monitor directly. My guess is that would be a simpler way to acquire the video image than snooping the processor bus, though I may be wrong. The data would still be digital either way, since it's a 1-bit video signal. Or maybe the RPi *could* do that directly - it would require sampling three digital I/Os at exactly 22.25 KHz, with enough processing time left over to do something useful with it.