The readtables and reader macros are entirely independent of Common Lisp compilers. The compilers work on the representation (that is, trees of cons cells) that is produced by the reader, which is after the readtables and reader macros have done their work.

If you are thinking of a Common Lisp compiler as working on a text file, you're thinking about it wrong. In Common Lisp, you can invoke the COMPILE function on a lambda expression that you cons up at runtime and that never is in a textual representation.

As a general rule, don't reimplement a Lisp except as a learning exercise (in which case, you're not going to reimplement Common Lisp; it's too big.)