Can you upload two file sets?
One being dithered to 24 bits and the other being dithered to 16 bits? That would be the most ideal solution.
If not have you tried emailing Bandcamp about this? Those guys seem to be pretty on top of things. I would not be surprised if they do indeed dither when truncating from 24 bits to 16 bits. Sure this eliminates your ability to choose the dither algorithm but I wouldn't stress about it much as long as they do indeed dither during 24 bit to 16 bit conversion.