Micah, Josh just pointed me at this issue, which is well-timed. I just ran into the same problem recently and implemented a solution I was going to submit a patch for. My problem was that I wanted to override the avro generic classes rather than the specific. My implementation is very similar to yours, only I changed the reflect factory to a ReaderWriterFactory and added an AvroMode enum to handle each case (REFLECT, SPECIFIC, GENERIC). The enum approach brings all of the code together like your AvroDataFactory, but keeps the handling for each mode separate. Each mode can be individually overridden:
It also cleans up some of the places that hard-code specific or reflect readers to use the right one based on the AvroType:
This is what makes it possible for me to override the generics correctly. A lot of places simply used Reflect because it is the most general... but that causes problems if you need to change specific (e.g., use a different ClassLoader) or generic.
Let me know what you think of the branch and whether it might work for your needs. Please ignore all of the unnecessary import changes, I still need to clean it up.