If no one else has a method to do it, you can metascript it. The following assumes that your attack roll is the 0th roll (ie, the first) of the message.
{& if 0 < {& eval}getDiceByVal($[[0]] >0 crit count){& /eval}}{& eval}!w --<character name> --You scored a critical hit. Would you like to roll Divine Smite? --CRITICAL HIT --Smite|<character name>|<Smite ability>{\& /eval}{&end}
Drop that anywhere in another command line to have it process. You will have to replace the <character name> with your character's name (or token name, etc.), and replace <Smite ability> with the ability on your character sheet that would fire the Diving Smite attack. Alternately, you can reference an attribute that would fire the Divine Smite attack this way:
--Smite|<character name>|@<Smite ability>
If the above detects a critical hit, it fires off the call to DiscreteWhisper, and you get a message in the chat like this:

Required scripts: ZeroFrame, APILogic, Plugger, DiscreteWhisper, and possibly SelectManager