0) There are two different major revisions of this player, according to its manufcturer (JLabs)
1) JLabs doesn't provide any public firmware download for either version.
2) There's no public information about the device's guts, including what SoC it uses. The FCC photos are too low-resolution to make out the chip markings, but _appear_ to show external RAM and Flash chips.
3) The needs of a UI capable of using that screen would seem to indicate that at least has the possibility of being able to run Rockbox with a nominal feature set.
So all that said, while I think it is probably _technically possible_ for a rockbox port to these specific players, unless you're volunteering to do the considerable reverse engineering and porting work yourself, it's extremely unlikely to happen.