I'd like to second the feature request to be able to service blockwise GET without the 65K limitation (or also, the limitation of needing to put the entire resource into RAM).
I understand how to use external_blockwise_store flag in the M2MResource constructor, and set_outgoing_block_message_callback, to handle storing a resource externally.
But I'd like an example (maybe by overriding "handle_get_request?") of how to serve up a file one block at a time. Or a feature added to mbed-client to do this.