Voltage level in an open drain bus (I2C)
A 'normal' diode (like 1N4148) would have a voltage drop of around 0.6V. The transistor might give you something like 0.2V in addition to that (check the datasheet). This could give you a total voltage drop of 0.8V, or just 0.6V. 

Here, you can find how TTL and CMOS will see this:


TTL: 0 to 0.8V is low, 2 to VCC is high
CMOS: 0 to VDD/2 is low, VDD/2 to VDD is high


So this could get tricky with TTL, and you might be more secure using a schottky or germanium diode with a voltage drop of 0.2V and 0.3V, respectively. However, it might just work with a standard diode, so just test it and you'll see if it works!