Dave, that's the stuff. The Starrett thread tools, like Holescreek said, come with "cone & vee" and full thread rolls. You're supposed to use both (need two gages) to measure a single thread.
Each gage is like $1500 or something. Then, a set of thread rolls is like $200 or something. Similar to the cost of a pair of thread rings, is what I remember.
THEN you'd need a thread setting plug to accurately set the gage.
It's really a high high high production setup... cool though, and they DO make one that DOES give you a variables data measurement on ID threads.
I like winger's idea of modifying an Indi-Cal.
I've used those things quite a bit and own 2 of them. With a .0001" indicator, you can actually get repeatable readings of +/- .0002" accuracy in a clean bore. You just have to be super super careful not to bump it.
There really should be an inside thread bore gage built in the style of a Sunnen but with interchangeable gage balls for contacts. Right? Would that be the best setup?
I'm thinking now of Mitutoyo's Digimatic bore gages, the kind with 3 cylinders for contact and tons and tons of measurement pressure :/ You can replace the cylindrical contact anvils on those things, and they have enough range to get in and out of common threads.