I’m not aware of an OpenIGTLink interface for Polhemus. Since cost of development, testing, optimization, maintenance, and support of a new interface would be higher than the cost of an already supported electromagnetic tracker (such as NDI Aurora or Ascension), probably most people end up buying an already supported tracker instead. Surgical navigation users may also end up choosing NDI instead of Polhemus, because NDI appears to be used in many more clinical products.
If somebody already has several Polhemus tracking systems or wants to use some unique characteristics of these systems then it would worth implementing an OpenIGTLink interface. Maybe a quick&dirty bridge can be put together in Python, which converts Polhemus VRPN messages (received by some VRPN Python package) to OpenIGTLink messages (sent by pyigtl).