Maybe we'll have something like a pop up asking if we want the reaction to trigger or not. But in this case it's a little bit useless to enable it before the end of your turn...
Really hope this TB is going to be more "interactive" for players and AI during other's turns. That could be a perfect mix between the classic TB games and the RTWP/D&D experience.
The Solasta demo pops up a yes/no dialog asking if you want to use your reaction, which might seem quite intrusive to some, as it stops the game. An alternative would be a transient dialog up for a few seconds "press to activate shield" or "press to acivate attack of opportunity", or something similar.
I think their design is because they didn't want to stop MP every time a reaction was possible, but I think a transient system could work well.