Sorry I misinterpreted the question. THis ping sensor gives a single pulse and its width is indicative for the distance.
Why not use an attiny13 - http://www.atmel.com/Images/doc2535.pdf
- and send the value in a binary stream (SPI) to the Arduino.
There are other "minimal" processors that could do the job too..
Don't know much about the ATTiny - but could you instead output PWM and use a cap hanging off the pin to "output" an analog voltage level...? Main question would be if there are enough pins/resources to do that...