Currently this library only works on certain boards, what about making it work on other boards too?

Since communication between MCU and NINA-W102 is carried out via a serial interface, I think in principle it is possible.

I think this would be useful when building a custom board connecting an arbitrary MCU to a W102 flashed with arduino/nina-fw.

I don't know details of this library so I apologize if I'm wrong, but for example, how about using a user-defined serial object if none of #if macros below match?

Example:

// Line 38~
#elif defined(SerialBLE)
#define SerialHCI SerialBLE
#else
#error "..."
#endif

SerialBLE is user-defined macro.
Define in sketch file.

// Sketch
#define SerialBLE Serial2
void setup() {}
void loop() {}

Alternatively, you may specify SerialHCI directly from sketch file without using a user-defined macro.

// Line 38~
#elif defined(SerialHCI)
// Already defined by user.
#else
#error "..."
#endif

// Sketch
# define SerialHCI Serial2
void setup() {}
void loop() {}