I think the lower case "b" in a circle is perfect since bytes use a capital "B" (i.e. 23 MB) and bits use a lowercase "b" (1024 Mbps). So the lowercase "b" would be the "bit" and the circle would be a coin. 
That's impeccable logic, and I agree with you that this would be an excellent symbol (if a symbol is needed). It's easily remembered, easily handwritten, and already present in Unicode.
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