This I'm 99% sure can be done via css. I applied "new" indicators after the title that differ from one another so I assume node icons can differ also. I'm sure someone else will jump in with a solution before i do but to cut a long story short, extra css can be used here with the subforum ID used to display specific icons per sub forum. This is CyberAP's subforum add-on correct?
Basically, if you use a code inspector (firebug) or another apply your ID though if memory recalls you'll have to overide the background-color with a background (image)
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