Can you react with a chosen champion from the champion zone?

Yes, you can react with a Chosen Champion from the Champion Zone, provided that the Champion card itself possesses the Reaction keyword.

According to the rules, the Champion Zone is a non-board zone (FAQ #1213, FAQ #3624). Units in the Champion Zone cannot be targeted by effects that target units on the board, and they cannot use their abilities while in that zone (FAQ #3843).

Here is the breakdown of how this functions:

1. Champion Zone Status: The Champion Zone is not part of the board. Units in this zone are not considered to be on the board and cannot be interacted with as units until they are played.
2. Playing a Champion: To use your Chosen Champion, you must "play" it by paying its summoning cost and placing it onto the board (your base or a battlefield you control). Once it is on the board, it functions as a normal unit (FAQ #3843).
3. Reaction Timing: The keyword Reaction (Rule 739) provides permission to play a card or activate an ability during a Closed State. If your Chosen Champion has the Reaction keyword, you may use that permission to play the card from the Champion Zone onto the board during a Closed State (Rule 739.3.a). If the card does not have the Reaction keyword, it must be played to the board during an Open State.

In summary, the Champion Zone acts as a holding area. You must play the card from that zone to the board before it can participate in combat or use abilities. You may only play it from the Champion Zone during a Closed State if the card specifically possesses the Reaction keyword.

Note: I could not fully verify this ruling against the retrieved rules.