I think that the answer is no, but before concluding that, I thought to ask the Ruckus brain trust.
I have an existing R750-based (x3) Unleashed network (mesh-enabled, but all APs are wired). I have added a R320 wired first and it joins the AP group without issue. When I disconnect the ethernet which should force it to RF mesh, it stays disconnected. So my strong suspicion is that it cannot be a mesh Unleashed AP (the config page for this AP does state that Mesh status is not supported). Unfortunately there is no documentation that I have found that explicitly states that (as there is for the R310 and H320). I just wanted to confirm that I will need a different AP to mesh Unleashed.