#define _H4X0R_CONVERT_ONE(arg)                  \
        ((union { unsigned long long u; void *v; }){ \
            .u = (unsigned long long)arg,           \
    }).v

Couldn't this be just

    #define _H4X0R_CONVERT_ONE(arg) (void*)(uintptr_t)(arg)

?

Also, thanks, now I can finally use

    void my_printf(const char *fmt, void* args[], size_t argc);
    

ergonomically:

    #define my_printf(fmt, ...) (my_printf)((fmt), \
        (void*[]){ H4X0R_VA_VOID_STAR_CONVERT(__VA_ARGS__) }, \
        H4X0R_VA_COUNT(__VA_ARGS__))

    int main(int argc, char **argv) {
        my_printf("int: %d, ptr: %p, str: %s, missing: %d\n", 42, argv, "Hello world!");
    }

    $ gcc test.c && ./a.out
    int: 42, ptr: 0x7FFF46AA3E78, str: Hello world!, missing: %!d(MISSING)

Funnily enough, the difference between passing ... and locally-allocated void*[] is basically who has to spill the data to the stack, the caller or the called function.