Your understanding is correct - you will not see SPO contested at a National event (and therefore not at the Runoffs).
SPO is a "catch-all" class for vehicles (I've seen pickup trucks running SPO) with modifications that exceed the limits of any other class. As long as the car meets GT-level safety equipment, it's pretty much run whatcha brung and hope ya brung enough. There are some limits (no nitrous, for instance, because it's specifically banned in all SCCA events), but it's pretty much wide open.
All that said, SPO is a very popular class here in the Southeast with everything from ex-Winston Cup cars, turbo Porsches, big-block GT-1 cars, a rotary powered MG Midget, a turbo rotary Datsun 510 and a lot of other stuff showing up for a given weekend. Gives everyone a place to race and makes for a good show with the variety it attracts.
Butch Kummer: Corner worker, (former) Solo I steward, SARRC Rep, Road Racer