Rockbox will down-sample to 16/44.1 before feeding bits to the DAC. Now go read post 10362330.
This is a bit oversimplified. Rockbox has to have a fixed output sampling rate, which is configurable (44.1k or 48k if you use Opus a lot). Internally, all of the rockbox code uses 32 bit samples (ARM processors are 32 bit so using anything else wouldn't be efficient). What gets passed to the DAC is whatever the audio driver decides to pass (it actually can take 32 bit samples if it wants).
I discussed this with the DX50 developer, and told him to test 24 bit output and see if the hardware actually support it. Basically any modern DAC will happily accept 24 or even 32 bit samples, but most do not actually use the extra information, so you have to test and see what comes out. If it can actually produce a different output voltage when you change the 17 bit in an audio sample, then it will be simple to support 24 bit output. If changing the 17th bit does nothing, then I suggested he use 16 bit output to save battery life. Most likely though the port will eventually support whatever the hardware is actually capable of.