Does exotic potassium exist?
First, a point that is more like a comment: I'm not sure that just because electrons get put in to d-orbitals (and thinking of crystal band structures in terms of atomic orbitals is of questionable value anyway) that it would not be a metal. Many elements (one could even say most elements) with d-electrons are quite happy as metals.

You might want to start with L.F. Lundegaard et al., Phys Rev B80 020101(R) (2009), where they do DFT calculations on potassium at different pressures. I will quote from their conclusions section: 


  The observation of the oP8 structure in K reveals it to
  have a pressure-induced structural behavior like that of Na,
  as well as having behavior like Rb and Cs as indicated by the
  observation of the tI4 and oC16 phases. The similarity to Na
  is unexpected in relation to the recent electronic structure
  calculations of Ma et al.$^{19}$ As noted, they found K, Rb, and Cs to have the same high-pressure structural sequence, and
  they attributed this to these heavier alkalis’ being dominated
  by d electrons arising from s-d transfer, whereas the ultrahigh
  pressure behavior of Na arises, they suggest, from p-d
  hybridization. Thus our results question this picture of a
  simple dependence of structural stability on d-electron dominance
  or p-d hybridization.