Logic level converter for H-bridge and UART for Raspberry Pi
Uh, I'm not entirely clear on the issue here. You do know you can simply apply a static pull-down directly on the raspberry pi IO pins? The rPi can easily drive a few kilo-ohms load. 

Just tie the output pin to ground with a ~5-50K resistor. That way, when the pi isn't set as an output, the pin will be pulled to ground, and when you do set the pins as an output, the output driver can just overpower the pull-down, and it'll work as normal.



For that matter, what H-Bridge are you using? The common L298 will work fine with 3.3V logic-level inputs, it's probable that the bridge-driver you're using will probably also be fine.