I just noticed your post.

There is no problem dropping 24V for an optoisolator. We do it regularly. I would put a 2.2K .5W resistor and a diode in series with the LED in the opto, and another diode across the LED in the reverse direction. Put a 10K pull-up on the opto collector and a 10uF across the opto. The time constant of this combination is 100 mSec.