It would be useful to allow specifying an arbitrary number of bits (and fill the remaining with 0). That would allow smaller flac files without affecting quality, since one could choose number of bits below noise floor (ie: 18 bits= 108dB). Another use is to test different dither settings shapes at evident low bit resolutions (ie: pass to 10 bits or even lower).
(Now I'm doing that with old and reliable CoolEditPro 2.1), but it would be better to do it with SOX.
You can use the dither effect with the -p option to reduce the samples to an arbitrary number of bits.
Thanks for your answer, it's useful, but still incomplete. Sometimes I don't want to use dither, so not being able to truncate to an arbitrary number of bits, is still missing.