David, in order to put clear what I was proposing to you in previous mail regarding to implementing plugin systems for numpy, please take a look at the attached tarball.
The plugins are in charge of implementing the action of generic foo() and bar() functions in C. The example actually implements two different plugins in Cython. There is also an include file with some support declarations and some macro definitions wrap_foo and wrap_bar
Next, in mymodule.pyx, I define python side foo() and bar() functions (calling funtins pointers in a struct via the macros wrap_xxx) and a use() for selecting the plugin at runtime.
Finally, the test.py script is pure python code showing all this in action. If you want to give a try, then install Cython, next do 'make', and finally 'python test.py'
Please note that I've implemented this is Cython just for convenience, it does not actually depend in any special Cython feature, and could be translated to pure C code with Python C/API calls.
IMHO, this is the easier and cleaner way to deal inside python with plugins written in low-level C. It does not depend explicitely on 'dlopen' stuff from your side.
Now tell me. What is in your mind that this is not general/robust enough as to have to explicitely deal with dlopen? It even works on systems with no dynload, provided that all is put inside the python executable.