In that case, readline was not a loadable module. A comparison that does use loadable modules is the FSF's GCC project. The FSF resisted implementing support for loadable modules in GCC for a long time under the belief that it would allow the use of GPL-incompatible modules. It was not until LLVM made it a moot point because GCC itself could be replaced entirely non-copyleft code that GCC gained support for this. Linux kernel module support analogously permits loading modules that are under GPL-incompatible licenses.
Note that I am not associated with the Debian project and therefore I was not involved in the discussion referenced here.
Another one I've been wondering about recently is the inverse— loading at runtime a GPL module into an otherwise BSD codebase.
ROS (robot operating system) runs into this with nodelets, which are shared objects that are loaded into a nodelet manager. Is it a GPL violation to supply a launchfile which specs the loading of BSD and GPL nodelets into a single running process?
BSD and GPL are actually compatible and can be distributed together. It's my understanding that the advertising clause is the bit that makes them incompatible, and that regular BSD and GPL code can be bundled and distributed together.
What else I got from the OP thread is that if you do this (lets just assume the two licenses are incompatible) then you are not the violator, since you haven't distributed these as one binary package, but the users might be (only if they go on to redistribute the pre-built confabulation of binaries/processes as one package, or even just in uploading them together to, say, a hosting provider.)
Note that I am not associated with the Debian project and therefore I was not involved in the discussion referenced here.