From my current workflow I am evolving I would use an external editor in a toolchain with Scribus as the last link in the chain.
Atom is my choice.
https://atom.io/Write your warnings posters as separate files with extension *.md
You can add conventional css styles.
You can concatenate markdown files where needed. Use the @import command to import files or objects.
Use Markdown Preview Enhanced package in Atom.
Export as image or pdf and import into Scribus frame.
There is a learning curve in understanding how to use Atom and markdown with Scribus. I plan to add a repository although so far Atom package Project Manager is good for different projects.
All of this this assumes that you are willing to experiment with a toolchain.
Alternatively you can do everything in Scribus if you choose to explore the options.
You might also look at ScribusGenerator.py script in Scripter to modify warning sign templates. Instead of the business cards example build some "warning sign templates". You can then have a csv file defining all your warning variables, even in different styles. In fact this might be the first option to try.