I must check the API for this (the docs are still incomplete).
The problem of not knowing the filenames still remains.
It's not that SABnzbd first reads the first article of each file to get the filename.
Filenames present in the API are guesswork upto the reception of the first article.
I hope you understand that we're not eager to do serious
maintenance on the downloader internals because XBMC expects 0-byte files
(BTW: great program, I have been using it for years).
For 0.6.0, nothing of this kind will be implemented.
We want to converse asap to a Final release.