Fact of the matter is that this is something that can't be done from WoW -- thus it can't be done as an addon.
It would be very possible to completely disable the caps lock key through a third-party executable, however tying it in so that it can be used as a cast modifier would be much more complicated (and technically against the TOS), unfortunately.
However: Making it so that chat is auto un-capsed if it detects that caps lock appears to be on should be fairly simple.
fOR EXAMPLE IF IT SEES CHAT LIKE THIS, i SHOULD BE ABLE TO WRITE AN ADDON TO CONVERT IT TO
For example if it sees chat like this, I should be able to write an addon to convert it to
^ this before sending it in whatever chat system desired.
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たしかにひとつのじだいがおわるのお
ぼくはこのめでみたよ
だけどつぎがじぶんおばんだってことわ
しりたくなかったんだ
It's my turn next.
Shakespeare liked regexes too!
/(bb|[^b]{2})/
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