I recently start a new opensource (and very small but usefull) project:
http://pfct4net.sourceforge.net/
 
It's purpose is to go beyond limitation of DllImportAttribute, calling any function pointer, and not only those returned by GetProcAddress() ....
 
the next release of CsGL might drop extension support and advice you to use this library instead,
it's while I ask you some feedback and if it looks ok ?
 
cheers,
Lloyd