Creating a case insensitive data type

could CREATE TYPE be used to make a case insensitive version of varchar? 
So that doing something like LOWER(username) = 'joe' could just be done 
like username = 'joe' ?

I want to try to avoid using CREATE TYPE with C extensions and using a 
new operator all over the place is not an option... How could I do this? 
Can I accomplish this via another method?

-Dave