Based on my current research, this functionality is not considered a priority within the Rockbox development community.
It sounds like it is possible (provided the device supports inline controls), but there isn't enough demand to warrant the limited resources of developers.
Inline controls are important for people who use their devices while engaging in active lifestyle activities (jogging, cycling, etc).
As a cyclist who listens to music in one ear during lengthy cycling commutes, it is not possible for me to operate the device itself safely while riding.
On the other hand, it is very easy for me to press a headset button to pause/skip/go back.
Based on what I have read, the recently released X3ii supports inline control through both BT and wired connections.
Rockbox has been ported, and my fingers are crossed that some consideration would be given to developing such functionality.
With that said, unfortunately, I imagine most resources are committed towards higher priorities.
Nevertheless - fingers crossed!