According to the TWI sections of the documentation here and here the TWI baud rate is variable through the whole range of 0Hz - 1 MHz. The implementation of the Wire library for the megaavr however explicitly hides that functionality in a hidden way.

I suggest removing the code that limits the baudrate to the 3 distinct values defined in the Philips I2C standard in order to be able to communicate with devices that do not match that standard.