Motor Thermal Curve

There probably is a method to do this but you must consider the legal problem first. The motor manufacturer has probably provided two temperature windows for this motor. One temperature range for the powered operations and one for the unpowered storage. If you exceed these temperatures you do so at your own risk!

Likely many components outside the approved temperature regions have very non-linear characteristics. Where and how they go non-linear is anybody's guess.

Have you not learned from the Challenger disaster?

What size motor are you referring to? Smaller motors tend to follow industry standards and the damage curve is supposed to be under the i²t curves you would find in a standard over load relay. If you are referring to a large and / or custom built motor, then all bets are off.

Why do you need to draw the thermal curve? Hot & Cold Time Constants are enough information with respect to motor protection.