My T420 was badly damaged earlier this year in a traumatizing accident. I no longer have a display, webcam, wifi, display cover, indicator lights (battery/sleep mode) or microphone. Even though my Nvidia nvs 4200M is still inside my system, when I use discrete graphics or optimus my system stutters really bad and if I go into a graphic intensive program (or game) my ThinkPad just shuts off (doesn't even get hot, it just ragequits) my ram, ssd, processor and the rest of my unit seems to be working normally. I am using an external display to access the system.

Would it be possible to connect a 15" (like t500 or w510) display to the t420's motherboard with the T420's 1600x900 40 pin connector? If so, is there any way to use 15" display with the t420's hinges?

Hi there...
Sorry to hear your accident with T420..... its a very nice machine...i have couple of T420s in use

Not sure what or how u using the TP (broken 420) .....
but as a FYI ... i have had few TPs (t60p + t61p) both these machines are kinda 'perm'tly' connected to EXTERNAL DISPLAYS for use. The teen's @ my house love it as they get to use the EXTERNAL DISPLAYS which is lots bigger than 14 & 15 inch screens. These laptops = also have STILL - AS "NEAR FACTORY displays" (IPS panels)....<<saves my IPS panels for lots longer>>
SO yes its possible to 'hook'up larger displays to TP's that have their screen issues ( use toggle F' keys ) ...or via Docking stations (like I have ) to BYPASS the laptops inbuilt screens.
I am not sure of that exect method you described (soldering to the MB etc) will work.... as am no MB EXPERT.... however I do did try that some yearrrr's ago(on a t43p) and that time that MB would boot up but mins later would SHUT DOWN with "over heat' alerts and logs reflected those kinda issues ........ so I gave up and connected a EXTERNAL SCREEN ,,,, and later TRASHED that laptop with a HAMMER !!

sorry not much good news ... but just sharing some old experiances and suggestions ...maybe others can help you .... as I know there are a lots of guys who like to do things @ MB level ......

cheers
mike