You honestly expect me to figure out how to implement a plugin system?!?!
Uh... you have functions, you have a functiontable, you take the address of the functions and put them in the table...
Amiga-like shared libraries offer plugin functionality for free :-)
I prefer the virtual DSP idea anyway, as it is platform independant and is not limited to this project
You may just as well compile the functions on the fly. Have them written in C/C++, invoke the compiler, load them.
Fast and portable. Yes, you need to ship with or rely on a C compiler, but so what?
The other approach would be to compile your own scripting/opcode language. Unless you want to allow for self-modifying filters (:-o), compiling them will always give better perfomances than interpreting them.