I finally got bored enough to install DevKit. (Which is very time-consuming to install, wow.) Once I got it up and running, it didn’t take me long to find the random NPC spawn table, WeightedSpawnTableRow, and it didn’t take me long skimming it to figure out how random thrall spawns work. For each artisan type and faction, there’s a four line table specifying the weighted chance of spawning a low, medium, high, or legendary (t1-4) thrall, and then a separate table for each tier showing what random spawns are possible.
For most artisan types, the odds are 56% tier 1, 37% tier 2, 6% tier 3, and 1% tier 4. So if, for example, you’re looking for one particular named armorer at the Black Galleon, where there are five possible named armorers, your odds of finding him or her are exactly 2 in 885 (per spawnpoint).
I haven’t found the tables for totally-random artisan, yet. But if it first rolls an even chance of alchemist, armorer, blacksmith, carpenter, entertainer, smelter, tanner, taskmaster? That would suggest that the odds of finding Grr Legbiter are 1 in 3,540.
There’s rare … and then there’s stupid-rare.