Can an opto-isolated input be analog?
In the context of the O.P.'s I/O box 

It comes with full schematic.  That could enable a little bit of hacking.
If I'm reading the schematic correctly, the opto-coupled inputs are connected to PC1/ADC1, PC2/ADC2, PC3/ADC3 on the ATMega.  If U9, which is the opto-coupler IC, is removed then the ADC lines can be connected directly to the outside connectors.  That would create 3 straight analog inputs.

Outside of the context of the O.P.'s relay I/O board.  
More generally.

Yes, an analog input can be opto-isolated.  This is usually done with specialized opto-couplers, such as LOC series.





(source: IXYS application note AN-107)

These types of circuits aren't neatly as common as digital opto-couplers, though.