Yeah. Somewhere north of 50 Ω (and certainly above 300 Ω), the amp becomes voltage limited. That's why the curves bunch up near the 9 V RMS limit. So I would expect 250 mW into 300 Ω, 125 mW into 600 Ω, 62.5 mW into 1200 Ω, etc.
Exactly where the transition between voltage limited and current limited is, is honestly mostly of academic interest. I'm sure I can develop a mathematical model for it and plot it for you, but I would rather focus on selling more HPA-1s at the moment. The bottom line is that you'll get 250 mW into 300 Ω at vanishingly low distortion and "around 1-1.5 W" into 12-50 Ω.
Another point I'm making with the graph above is that the amplifier is rock solid even with 12 Ω load.
Tom