datitisev Turns out that's not true. By default, I'm able to use BBcode inside of BBcode content. It's the attributes part which doesn't work. And, I tried various versions of the example you provided, but, I just don't think it's possible for extra BBcode to be inside of BBcode attributes.
Long story short, things seem to work fine. You can add extra BBcode formatting to this extension's BBcode, as long as you are only doing it for the content.
Here's an example. Without doing the XSL stuff, I can use BBcode inside of BBcode to make the word "secret" in this extension's spoiler, bold:
So, I still stand by my original statement of, the exact thing @Somyrion brought up could be possible because in coding, most anything is possible - but - it's not the way things are meant to be. By default, BBcode works inside BBcode - but, by default, BBcode inside of a BBcode attribute does not work, and I'm not sure the XSL route would work either.