For example, how hard would it be to replace the LEDs on the bezel/lid
Depends on the model. I don't know about the T4x, but for the X4x and X6x they're small LEDs that are surface mounted to a board within the lid assembly. With the right gear I imagine it'd be pretty easy to remove them and attach new ones to the traces -- but first you'd have to figure out the electrical requirements...
As far as the keys, well... that depends on how fine a grit of sandpaper and how much free time you have.
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Current laptop: Lenovo ThinkPad T420 (running Debian Wheezy) - kestrel [pending retirement]
Current workstation: IBM Intellistation 9228 (running FreeBSD 9.1) - blackbird