Mmh, I'm not used to optocouplers but why not. Now let me guess... you do have a CNY17 in stock, don't you ? :-). Well, I'm probably not the best to comment on that solution, but from a newbie point of view, this doesn't seem incredibly simpler than using the MAX I think...
I'm asking because if the max232 is not needed from the HU to the PC I'm thinking to replace the transistor and the max232 with an optocoupler and a diode (since the optocoupler wouldn't stand more than 5v of reverse voltage).
Note the apparently reversed connection of the optocoupler: the transistor will be open with a positive TX (hence a 12V via the internal pull-up of the HU) and closed with a negative TX (hence 0V to the HU).
Edit: of course a resistor is needed in series with the optocoupler's led in order not to fry it. Around 2K should do.