I can't answer you question directly regarding low speed buffet at altitude so apologies for that.
If you have an FMC with the hold page function then you could use that to give you a sensible minimum speed to fly in the cruise at high altitude. Select PPOS but don't execute. Note the speed it generates and don't fly below that, thus remaining on the correct side of the drag curve.
If you don't have an FMC then use the FCOM tables for holding. They will tell you amongst other things a speed to fly based on weight and altitude. Again a safe and sensible minimum speed.
Good luck and let us know if you get anything firm.