Does taking keys off of a MacBook keyboard void warranty
Assuming you signed the AppleCare plan on the US, this is what the legal document states:


  4. What is not Covered?
  
  (ii) Damage caused by (…) (b) accident, abuse, misuse, liquid contact, fire, earthquake or other external cause, (c) operating the Covered Equipment outside the permitted or intended uses described by the manufacturer, or (d) service (including upgrades and expansions) performed by anyone who is not a representative of Apple or an Apple Authorized Service Provider (“AASP”);


I'm not a lawyer, but abuse and misuse may be applied to removing keys.
You may get away with it if you present it as an "upgrade", and you are an Apple Authorized Service Provider :)