Sweet, I missed this thread. I'm growing a bunch from seed.
As for the tap root, judging from the soil it's going to be a big and deep one. I can recommend digging down in stages, you might not find roots in the first foot of depth, and all of them just below. So instead of digging in a typical V shape underneath the tree, I'd dig a trench straight down and work your way inwards from there to find the biggest root mass. Rigida sparked my interes because of their backbudding capabilities, so a super long taproot would just be a trunk to work with; not that much of an issue compared to any other pine.