For the 24V serial input TI has an interesting parallel to serial converter

http://www.ti.com/lit/ds/symlink/sn65hvs880.pdf

It handles 24V inputs, runs of 24V and even will regulate to 5V. It also has LED drivers to indicate the status of the inputs. If you must have isolation, then you can isolate after the data is converted to serial and so reduce the number of isolators required.

You can similarly reduce the number of optos on the output using a serial to parallel converter, especially one that can handle the 24V. There used to be a much greater selection like the A6810 (TL5810) from Allegro, but manufacturers seem to be pulling out of the market. The ST part STP16CP05 would be ideal except that it only goes to 20V. The TI part TPL9202 is also deficient when it comes to output voltage. Maxim also has a part, I believe. I have not yet come up with a really good replacement for the TL5810, 5812 etc.

-Aubrey