Looking at this issue, it seems that the API needs to be extended with the signature mentioned in the first comment; then the various implementations (there are three listed above) need to be updated to match it.
WritableSerialization and JavaSerialization look pretty straightforward at first glance, as their corresponding RawComparator implementations (WritableComparator and JavaSerializationComparator) already themselves have factories.
AvroSerialization I think is a bit more complicated – the schema needs to be read in from the metadata, then a comparator instantiated over this metadata. I'm not as familiar with the avro classes involved, so I'd need to keep digging to figure out where exactly this all lies. Doug?
Jacob, I was actually looking at trying to get this working this week myself – I finally freed up some room in my schedule and I figured a crash course in Avro was overdue. Let me know if you feel strongly about tackling it yourself though.