Has anyone created a clever system for sharing reader macros?
as Fare mentioned, there's named-readtables, but we had to deal with this before it was implemented (plus i think it's still not 100% yet regarding infrastructure integration), so we do this:
shadow cl:in-package with out own macro that takes a readtable from the package and sets it up while compiling. see hu.dwim.common and hu.dwim.syntax-sugar
and to support working through slime we add our own entries to swank:*readtable-alist*
it can be made to work, but few people took the effort, so it's not a well trodden path.