So my dealer got back to me after running this to ground and told me that he was mistaken -- the prinsu rack would not fall under Lexus's warranty. Only the prinsu manufacturer's warranty is available, which is obviously what you'd get buying directly from prinsu. This makes the added cost of buying it through Lexus not justifiable IMV. So I'll be just buying my own after-market rack after pick up.
The AAP catalogue is more limited than I've seen suggested elsewhere. But regardless, if the non-Lexus parts won't be covered under the Lexus warranty it doesn't make a ton of sense to me to pay for the Lexus markup and labor, unless you just want it done all in one shot for convenience.