I tried using square brackets like this:
{{ [my_variable] }}
... which is not working. The square brackets are only working together with other text in the curly brackets:
{{ some_other_text.[my_variable] }}
Thanks!
Solved it with a hook / plugin as described here:
https://github.com/gemfarmer/jekyll-liquify
... still wonder why {{ [my_variable] }}
is not possible in liquid/jekyll...