Great feature! I am glad to see you plan to use the existing imageio framework for transferring data.
Will you allow export of VMs from a particular snapshot? I guess that's how you'll have to do it if you want to support export of running VMs.
I think you should definitely have a comment in the ovf to indicate that an OVA was generated by a oVirt. People will try to use this new feature to import random OVAs from who knows where. I'd also recommend adding a version to this comment: <!-- Generated by oVirt version 4.1.2 --> or perhaps even a schema version in case you need to deal with compatibility issues in the future.
I agree with Yaniv Kaul that we should offer to sparsify the VM to optimize it for export. We should also return compressed data. When exporting, does it make sense to cache the stored OVA file in some sort of ephemeral storage (host local is fine, storage domain may be better) in order to allow the client to resume or restart an interrupted download without having to start from scratch?