Actually, I don't think this is even theoretically possible. Even if USB satisfies the bandwidth requirements for a monitor (and that's a big if), there isn't a standardized device class.
There's something called Monitor Control Class but that's as far as it goes. Quote from the specs:
The Universal Serial Bus Monitor Control Class focuses on the management and control of monitors. USB
is not used to transfer the information actually displayed on the monitor. USB’s available bandwidth is not
sufficient for this task.
(emphasis mine)
Therefore, we would need to 1) design a protocol, 2) write the driver for the DAPs, 3) write the host-side drivers. I think this goes well beyond the scope of Rockbox.