Let’s use a global variable to keep track of the most recently discussed character.  We’ll just have Mention remember the most recently discussed character in the variable They.

Mention They: they 
Mention (Character ?c):
  ?c/GivenName+FamilyName
  [set They = ?c]
[end]
Mention ?anythingElse: [Write ?anythingElse]

initially: [set They = nobody]

The first line here says that if you call Mention on whoever is the current value of the variable They, just print the pronoun “they”.  If the character being mentioned isn’t the character in the variable They, then we move on to the next line, which says if they’re a character, print their first and last names, and then update They to be the character we just mentioned.  Now, if we say:

Introduce ?who:
   ?who is ?who/Age years old. ?who are a ?who/Occupation.
[end]

And run [Introduce diana], it will print “Diana Ratcliffe is 34 years old.  They are a titan of industry.”

Unfortunately, we’ve now assumed that Diana’s preferred pronoun is they.  A better approach would be to let the author specify preferred pronouns for the different characters.  So something like this:

Mention They: [PreferredPronoun They ?pronoun] [Write ?pronoun] 
Mention (Character ?c): ?c/GivenName+FamilyName [set They = ?c]
Mention ?anythingElse: [Write ?anythingElse]

initially: [set They = nobody]

PreferredPronoun diana she.
PreferredPronoun bill: they.

This says that when we mention They, we just generate that character’s preferred pronoun, whatever it might be.  Now it will generate “she” for Diana and “they” for Bill.