As far as I know, overriding a default dissector is not possible, or more precise not necessarily very easy. But what you could do is, to implement a generic heuristic entry point in the dis dissector, which then in turn you could use to let your custom plugin take over the dissection from the dis dissector. Take a look at the eth dissector and the heuristic dissector entry point.
I did it this way in the beginning of the openSAFETY dissector with the epl dissector, and it works just fine.
If you should keep a separate plugin and not implement the same features in the main dis dissector is a different discussion.
regards,
Roland