doesn't sound like madness to me.
maybe because i'm trying something exactly like this, on my old (and dead) acer 5050 laptop.
i'm struggling with some things, though, so, i'm gonna tell you what did i do, and i'm hoping to have someone giving some hints and tips on what can help go through this little project.
so, the idea was basically getting the old and broken motherboard taken out of it and replaced by a raspberry pi 3b, using the lcd, keyboard, touchpad and buttons. on top of this i still might add a 4 ports usb hub (mainly because two usb ports are very tight under the keyboard and might not be possible to be used at all, and to keep the laptop sockets for usb ports still working, just for the fun of it) and a usb WiFi dongle (i can explain that latter).
what do i have in the laptop case, up and running so far:
1. the lcd is working, connected to a board i bought on ebay (they're fairly common these days, you just need to pick the right one for your lcd specs);
2. the raspi is running on a Samsung 64Gb Evo+ class 10 micro SD card (i know you can run it off smaller, slower and cheaper cards, but speed and sizer were important for my project) - i have Kali installed on it (you can chose a quite fair different set of OSs that run on raspi);
3. got my raspi to be powered by the LCD driver board - i am aiming at having this looking and feeling like a working laptop;
4. as a temporary solution, i'm using a wireless keyboard/mouse.
what i'd like to have working but for whatever reason isn't so far:
1. the touchpad - got the wiring all done (as it's supposed to be a ps/2, but as they're directly compatible i wired it straight to usb). i don't get it to be detected anywhere - my desktop (win10), laptop (win10), raspi (kali). I've access to several models from synaptics, i've tried them all, none of them actually works - i really need help on this, as i'm running out of ideas (besides going GPIO and do my own code/driver to run it. If the touchpad itself gets to work, i'll be cutting the part of the (mouse) buttons off the old motherboard to get them to work with the pi too;
2. the keyboard - i didn't have any usb keyboard that would use the same number of pins on their driver, so, i'm now waiting for a small keyboard w/ case for tablet that i've ordered from ebay, my hopes go for it to work, with some remapping on the keyboard probably being needed;
3. cut the old motherboard (again) to get some more buttons from the laptop case to work - this time for the lcd driver buttons - not really necessary, but a nice to have;
4. reuse the lid switch to turn on/off the screen (easy);
5. reuse the power button from the case to actually power up both - the lcd board and raspi (relatively easy);
6. fit a decent lipo (i know, they all seem to explode these days
) inside the battery case and actually have it powering up this 'system', adding a circuit board to handle the charging too (one can only dream, no, the laptop is a 14.1", there's not that much space for everything).
well, there would be still a lot to write about this, but this is already too long.
what do you (all) think? is there any hope to make any of this work? - i'm focusing on the touchpad at the moment.
thank you a lot for reading!