Right... the file gateway is limited in this way. Have you tried using the file connector on the camel gateway? See the camel based quickstart.
Search the programmer's guide for the string "file-filter-class". It allows you to inject your own java.io.FileFilter.
That being said, we've discarded the default file gateway class and gone the "scheduled invocation" route that you noted above, because we needed more functionality from the file gateways than was possible with the default implementation.