Down this path lies madness. Now the user has to deal with the difference between YAML values and Jinja values. Can't really reference YAML data set in other files, or other places in this file.
It's a tangled mess. Please don't do this to your users.
Maybe use tcl? Guile? Some real language that lends itself to making a clean api/dsl for what you're trying to do.
I agree that YAML block style as on lines 22, 23, 24 should be eliminated (I will probably prevent my framework from parsing this). If you use an unescaped { or } it should signify that you are using Jinja2.
Similarly, I'm no fan of the {% sets %} at the top - it's a code smell.
Apart from those things, though, what you linked to seems easy to read and understand to me.
I'm absolutely positive that Tcl or Guile (or python) would create the potential for bigger messes than what you just linked to under similar circumstances. Simply being turing complete is enough for that.
Down this path lies madness. Now the user has to deal with the difference between YAML values and Jinja values. Can't really reference YAML data set in other files, or other places in this file.
It's a tangled mess. Please don't do this to your users.
Maybe use tcl? Guile? Some real language that lends itself to making a clean api/dsl for what you're trying to do.