I feel like a truncated 16-bit file is still a perfectly acceptable back-up, I've done both truncated and analog back-ups into my jb3, and I prefer truncation.
Sometimes they can sound very good. Saturday I did a 2 channel mixdown (from 6 channels) on the DEVA and fed that to the archivist of Oslopo for their set at Harvest Moon Festival in Austin. The DEVA only sends out a 24-bit AES signal out the digital output. The Tascam DA-P1 he used truncates the additional 8-bits and we listened to some of it later (I listened in the field with my Etymotic ER-4), and it sounded just fine.
Will the 24-bit file sound better? Yes, but as a backup or even a master depending on the source and other factors, a 16-bit truncated file sounds just fine.
Without dithering the next best way to get from 24-bit to 16-bit in the digital world is to use 16-bit floating point, processed in this manner will sound pretty good, and usually as good as any of the dithered files. That said, you have few devices taking in a 24-bit file and doing much more than just chopping off the LSB 8-bits.
Wayne