Regarding the stricter separation, Racket and its module system have nicely clear concepts of phases. (I really like how the Racket module system has been used these to benefit the language.)
But, though I don't know current Racket internals, I'm not sure those need to be a barrier to monkeypatching that approaches CL.
Other than optimizations that make some debugging info and dynamic changes difficult-to-impossible, if the system had a mode to disable these optimizations (or to maintain some information in parallel that permits a cascading unraveling of optimizations affected by a change), I'm not aware of a fundamental reason the system couldn't support more loose poking at things.
Though, one implementation complication that Schemes do have is first-class continuations.
But, though I don't know current Racket internals, I'm not sure those need to be a barrier to monkeypatching that approaches CL.
Other than optimizations that make some debugging info and dynamic changes difficult-to-impossible, if the system had a mode to disable these optimizations (or to maintain some information in parallel that permits a cascading unraveling of optimizations affected by a change), I'm not aware of a fundamental reason the system couldn't support more loose poking at things.
Though, one implementation complication that Schemes do have is first-class continuations.