First, thanks to herveus, dragonchild and Jenda for your comments, they allowed me to progress on my problem

Regarding XML, I have made a simple test, by using simple print, and XML is finally not the solution for me: some of my outputs contains '&' characters, and I can't escape them as I want to keep my outputs untouched.

I have also looked at structured text solutions, like wiki format or others, and the conclusion is that they don't fit my needs. In fact, I only need a way to separate outputs taken from my 3270 sessions, with only the command issued to get them. As 3270 screens are limited to 80 characters, I will use line separators of 81 characters, containing the information required to understand what is logged after. This is pretty simple, but will fit my needs. And as mentioned by dragonchild, putting the code to write and read the outputs will allow me to enforce the format, and make it extensible.