1. First you need the 1-dB compression point. So copy/duplicate your PA circuit in the same schematic.
2. Perform a HB simulation and get your gain curves for fixed loads.
3. Write equations in the results page for the 1-dB compression point as the difference between the small-signal gain (in dB) minus 1.
4. Re-use the value you got (of the input power at 1-dB compression) as the new available source power in the original circuit.
5. Re-simulate the HB for the load-pull.
Of course that is valid for as much circuit duplicates you can afford for each load. If you use 10 duplicates, that would give you a nice rough contour.
I have not found a more clever way yet !
Good luck