Your idea to write to the reminder of a character seems viable. Perhaps just a collection of keywords in the reminder? Then, each character bio could contain a description + context on what those keywords mean?
Another option could be something similar to the Strict Game Master that uses code to track things like inventory and skills.
Yet another could be leveraging /mem. That is in chronological order (I think) so maybe you can add entries there?
This page on Custom Code does a great job giving all the important info about the Json data of a character.