Zip Me Impressive is tested N/N.
There is no "dormant" state of HYPP-- the horse either inherited the defective gene, or it did not. Some horses who have tested positive (inherited the gene) apparently have few to no symptoms, but that does not change their positive status, the fact that they could start having symptoms at any time, and that they can pass on the defect.
But I will repeat-- Zip Me Impressive is N/N-- doesn't have HYPP, cannot pass it on.
Zippy is indeed a worthy choice for the right mare-- he earned the title of World's Best Appaloosa, and has 50 Top Ten World and National and Year end titles, Three bronze medallions and 14 ROMs.
Here's a video of him in a trail class--
And his web page-- http://www.appaloosa.org/zippy.html