Did it! Success... and it was pretty easy this time (as I showed them photos of the use of the plates on previous Minis). The bad news is that, dang, the price of "vanity" plates has gone up a lot here in Oregon. It now costs $214 every two years (and that's $100 more than regular plates).
What's more, they offered to have me register the Sprite as special collector car, where they give you period-looking plates (but with letters and numbers, and no choices allowed)... and THAT only requires that you register the car ONCE (and never again - which will save hundreds of dollars in just a few years). So, when renewal comes up in 2 years, I may retire the old "Boink" plates (and frame one).
The photo is a little washed out (as I brightened this night shot at the expense of flooding the plate - which as an "Oregon Trail" theme they don't use anymore).