MarkT:
... the I2C library is careful to avoid that and drives them as open-drain outputs (never actively pulled-up)...
MarkT, are you sure ?
I wrote a whole section here : Arduino Playground - I2CBi-directionalLevelShifter
The Wire library (mistakenly) enables the internal pull-up resistors. But with external pull-up resistors of 4k7 to the 3.3V everything should be fine.