Have a plugin which could turn the unfinished modal for example into a wireframe look & feel and then when you have the right icon, turn it off again.
I remember back in the days Expression Blend had some functionality to apply sketchy lines style (based on Bill Buxton's book - Sketching User Experiences).
Also, the prototyping functionality is still rather basic. Axure is much more powerful and ideal for the early sketching and testing phase.
But I'm sure Figma will have more sophisticated prototyping features in the future.