to disable the built-in I2C pullups on the MEGA one has to patch the file
C:\Program Files (x86)\Arduino\hardware\arduino\avr\libraries\Wire\utility
by outcommenting the lines
//activate internal pullups for twi.
as this file is located in a subdir of \avr\ and there is no such file in either different directory, e.g. \arm\ ...:
are there internal pullups also in the DUE?
in case they are: how can those be disabled then, too?