Using plain LuaJIT you can extend Trisul's functionality in a number of ways.

Skeleton Lua scripts you can copy and fill out

Bro IDS scripting vs Trisul scripting

How to structure your scripts

Links to step by step tutorials

Tutorial 1 : A Hello World

Tutorial 2 : A packet length counter

Trisul script to use UA-Parser regex to track HTTP User Agents

LuaJIT performance tips

Explains the Trisul Google QUIC protocol analyzer script