Menwiz uses only the following methods, that are the same in the two libs:
lcd->init();
lcd->setBacklight(HIGH);
lcd->setCursor(x, y);
lcd->noCursor();
lcd->createChar(0,z);
lcd->print();
lcd->write()
The lcd object is created in the sketch and not in the library (the labrary stores only the pointer to the object), therefore there should'nt be a problem. I did'nt test as I have not the lcd in my hand.
I made a small change in the menwiz.h file (the new version in github in minutes!). You will find the #define I2C. If you comment that line, the compiler include the standard LiquidCrystal lib instead of LiquidCrystal_I2C. I did'nt test it yet. If you can, please let me know.
Better solutions are welcome!