Sometimes I wonder if the wiki from that thread isn't based solely on one or two nice agents. An extension of an inventory based theory would say that "I class requested" should come after instrument upgrades at the gate (loosely assuming R > I > R-secret-CPU-bucket).

I would be willing to bet that the system doesn't have a place for I-waitlist at the gate, and an agent has the choice of force-coding it as an (instrument-supported?) upgrade with fare class X (we know CPU from X is a valid state for the system since CC and GS can do this), or coding as PR-1. Given how many agents refuse to do PR-1, I have a healthy amount of skepticism that it is policy to do so.

OP - at this point I think your options are HUCA until you find an agent who knows what PR-1 is and will do it for you, or just sit tight. You may very well have to fly this one in Y; it's a risk of doing the waitlist strategy. (This is assuming you have checked in and appear on the Upgrade Standby List).