Virtual RAM on a DAP doesn't make sense at all. The Rockbox code is loaded and run from RAM, and all remaining RAM* is used to buffer the audio tracks. As those tracks are read from the drive it doesn't make sense at all to swap them back to the drive again -- you need to read them from the drive, so it simply doesn't make a difference. There is no need for "more RAM" to run code.
* This is not completely correct as there are also RAM sections used for loading parts of Rockbox that are replaceable like plugins. However, IMO this simplification is sufficient here to understand the reason why virtual memory doesn't make sense at all