When Dylan went from prefix to infix syntax the reader macro indirection got all the more elaborate: what started out as a trivial reader macro that treated colons differently started delegating to a full-blown lex & yacc style parser instead.
All the better, most of this was achieved modularly: Lisp editor buffers and Lisp listeners would run happily alongside Dylan editor buffers and Dylan listeners, Lisp code could still compile load and run alongside Dylan code, and Lisp classes could interoperate cleanly with Dylan classes.
There's no question that Common Lisp and its predecessors are some of the most formidably adaptable and hackable programming environments ever created. Only Smalltalk really comes close, and they have complementary strengths I think. Standard macros, reader macros and a comprehensive MOP are some of the key things that stand out in Common Lisp's favour in that hackability comparison.