How significant are hysteresis losses in iron wires with RF currents?
I have doubts that hysterisis loss will count for much because most of the magnetic flux lines are around/outside the conductor and barely cutting through it. Yes, there is greater flux density up close to the wire but this is also the mechanism that makes skin conductivity (aka skin effect\$^1\$) so bad in iron at any appreciable frequency and, because current travels at the surface of the iron, the flux lines will probably never be totally contained within the iron meaning there is a significant air gap and B will be low hence hysterisis loss will also be low.

\$^1\$ Quote from wiki - 


  Skin depth also varies as the inverse square root of the permeability
  of the conductor. In the case of iron, its conductivity is about 1/7
  that of copper. However being ferromagnetic its permeability is about
  10,000 times greater. This reduces the skin depth for iron to about
  1/38 that of copper, about 220 micrometres at 60 Hz. Iron wire is thus
  useless for A.C. power lines. The skin effect also reduces the
  effective thickness of laminations in power transformers, increasing
  their losses. Iron rods work well for direct-current (DC) welding but
  it is impossible to use them at frequencies much higher than 60 Hz. At
  a few kilohertz, the welding rod will glow red hot as current flows
  through the greatly increased A.C. resistance resulting from the skin
  effect, with relatively little power remaining for the arc itself.
  Only non-magnetic rods can be used for high-frequency welding.