I haven't heard of any yet.

It would be unlikely considering they're numbers but who knows. Numerical equivalents to letters is the only option I can see, but most words would probably produce a string of numbers more than the maximum a level code uses.

A couple to try would be 00001471, 14710000, 17000000 and 0000017, if the maximum is eight. I can't remember.

I haven't heard of any yet.
Highly unlikely! As Team17 had to fit the game into 50mb, they wouldn't go hiding maps!