I would avoid functions with variable arguements. Although it can be done in C, the overhead is huge. Have a look at the library source for printf for an example.

I would recommend forcing all functions to take a pointer as an arguemnt allowing you to pass a pointer to anything you like. Casts will be required, so for example the funcion prototypes can all specify void * as their arguement, as in your example.

Re-coding a function that takes no parameters to take a dummy pointer is simple enough.

As this all looks like the underpinnings of RTOS like features, I would look at things like message queues to store the complicated data structures.