[racket] Remapping default character behavior in make-readtable

The make-readtable function allows me to remap individual characters
to new behaviors, and to remap the default behavior of unmapped
characters to new behavior.  It also allows me to remap individual
characters to the behavior of another readtable (such as the default
readtable).  I cannot, however, remap the default behavior of unmapped
characters to the behavior of another readtable.  (The same
restriction appears to hold for "dispatch macro" mappings.)  Is there
a way to do this that I have missed?  If not, could this ability be
added?

Carl Eastlund