Is it permissible to place the right hand over the left during prayer?

If you place your right hand over your left during the Iqamah, not as an act of Sunnah or worship, then there is no harm in it. However, if you do so as an act of worship or Sunnah, this is not permissible due to the lack of evidence for it, and the principle concerning acts of worship is that they are Tawqeefi (i.e., require textual evidence). Placing the hands on the chest, like in the posture of a prayer, is specific to prayer and not to the Iqamah.