A link to the site would be helpful, to know what the (currently FREE) app can do for Nikon:
Sofortbild - Mac Tethered Shooting
This blog note also tells the story about "other camera brands" and why he is Nikon-only right now:
Sofortbild FAQ #3: Canon, Sony, … Support in Sofortbild | Sofortbild
So, even though that blog note was from 2010, like many shareware/donationware developers, the author of the product would need both the tools (a camera control SDK from Pentax, and a Pentax camera!) and the financial reason (a market that is willing to pay for such software, unlike the Nikon folks getting it for free) to do it. We Pentaxians really do not have another option, so there is a market, but... there is no SDK available from Pentax for the author to know what to do and how to do it!
Sadly, considering how slowly PKTether has been progressing
without an official SDK (like Nikon and Canon both provide), it is really doubtful that this skilled developer will branch out to add tethering for a "black-box" camera system like Pentax. Still might be worth asking, as you say!