What is the problem for less than 1/2W?
The 4N32 works fine with 1.3 mA i used a bridge, an 150K (1/2W) resistor and a 4N32 in hundreds of circuits for testing AC 220V.
Then the only work to do is change the resistor for a "dinamic resistor" for testing 10 to 220 V.
If you change the bridge for a single diode you have less than 1/4W. Is a problem less than 1/4W?. Do you need a big heatsink? Or none heatsink?

I think that "plenty" IOs is more than 32 lines i usually use the '537 and i must expand ports because i need more! (depends of the aplicantion)

The 'forget' may be is an unespected hang of the sistem. The Murphy's law say something like "If something could be wrong, it will be wrong!"