Depending on what want for "lock down", this or something like this could work: you are essentially defining a single outbound communication path. In a way, your scenario was one of the reasons behind this experiment.

I'll take a look a the overflow thing, although I'm not sure if I will be able to fix it: I do have an image at the start which is an alternative to the text-based drawing, so nothing is lost. I use my own blogging solution that is essentially Texinfo (https://interlaye.red/Texiblog.html) so these blocks are the result of using an @example block (which is then converted into a preformatted block). I'm not sure this can be improved, apart from (as you said) using alternative images.