Reactions just need to be an IF - THEN prompt based on triggers and/or replenishable hidden buffs. For example add a hidden IF-THEN check to finesse weapons, that adds a Defensive Duelist buff to a character with that feat at the start of every round, and when ever that character wielding a finesse weapon with that feat is hit with a melee attack X times per round depending on amount of available reactions etc, be prompted with whether or not they want to use their reaction to benefit from said feat. That'd probably be the at-glance approach I'd have as a modder at least, but can be made a little cleaner with low programming access to the engine and game data itself. Reaction can't be actions, because... Well, those are two different things in D&D.
