No. Using a DAP as an external display screen would require a client side application and a server side application (one to send the data and one to display the data). It would also require us (Rockbox) to have a working USB mode of our own so that we could send data back and forth between the two programs. Rockbox does not currently have a working USB mode of its own on the Sansa.
The main issue, outside technical feasibility, is the fact that Rockbox is an audio player firmware. Though you are welcome to write a plugin to do this once Rockbox has its own USB stack.