Salsa do a seat clamp with external threads
The rock lobster has an brazed on seat clamp too I think – forward facing just to add insult to injury- that, cowled drop outs, no canti studs… its pretty much the most perfectly evolved rack-proof frame. I’d be tempted to say get a trailer instead but the dropouts might rule that out too.
Another thing to watch with a hard tail is if you’re a taller rider the short stays on an MTB can mean you keep catching the heels of your big feet on the panniers. Racks are usually proportioned for the longer stays on tourer
You might need to do things more bike-packing style with seat, bar and frame bag instead of panniers.