I know Attila (among others) makes heavy use of custom readtables. I wonder how he currently deal with those? (I've cc-ed Attila. :-)
there's already a package->readtable mapping in swank.
we use a custom hu.dwim.common:in-package that also sets up the readtable (our asdf system class makes sure the default package is hu.dwim.common instead of cl-user or asdf-user when loading a file, gory details in hu.dwim.asdf).
we also have a (def package ...) that allows for specifying a readtable setup form for a package that is used by the custom in-package macro, and also updates the internal swank mapping when the hu.dwim.def+swank asdf system is loaded.