I have a similar desire; in many cases I am using whatever images are colocated to serve this function, but as I write more blog posts and pages I find the want for something to make my links a bit more personalised.
I would love for it to be a simple templating language, like Markdown but with a renderer that produces images, or perhaps to take some inspiration from crates.io’s use of typst templates.
I suspect with some clever use of the existing Zola metadata and taxonomies this could be made both simple to start out on and flexible to customise.
Alternately, as this isn’t something you’re personally interested in @keats, perhaps a simple plugin/hook system could be worthy of consideration? Then one could have, say, a before-page hook (perhaps just a script or executable that gets called out to) which gets the page metadata and output path, and can then do some work and inject its own information back into the page generation process.
Obviously there are people who’ve worked around this by using off-site generators, but this is undesirable if your goal is to have a wholly static, self-contained site and not potentially depend on third parties.
Just for completeness, here’s another request for similar functionality: Generate Preview Images , and see also Run commands