Is voluntarily revealing a card from your hand considered proposing?
You can reveal your hand and cards at will. It doesn't count as casting.

3.12 Hidden Information
Hidden information refers to the faces of cards and other objects at which the rules of the game and format do not allow you to look.
Throughout the match, a draft, and pregame procedures, players are responsible for keeping their cards above the level of the playing surface and for making reasonable efforts to prevent hidden information from being revealed. However, players may choose to reveal their hands or any other hidden information available only to them, unless specifically prohibited by the rules.