Yes. You can look in index.html and play.html files in the Standard and Classic templates and you’ll see things not in the docs like OTHERCREDITS and INBROWSERPLAY in Standard/index.html. And you’ll see some of the things documented aren’t in the templates like TIMESTAMP and DATESTAMP.
But you can make up your own arbitrary placeholders in website templates and whatever value you put for them in ‘(manifest).txt’ in the interpreter template will get substituted.
And neither of these things do you much any good since you’re looking for a way to populate it via the Release statement in the source code, and so far as I know you can’t populate an arbitrary value that way.
Personally, I think it’s a kind of wonky design decision that Inform 7 calls on you to specify all of that in the source code, and that it’s the sort of thing that’s better suited for a configuration file of an external build tool. And @zarf has written such a tool, ifsitegen.py.
But it doesn’t have built-in support for arbitrary placeholders. It’s not hard to add another placeholder to it, but you’d need to add two fields for the link text and the filename and then either further modify the code to copy the file into the release folder or remember to do it manually and by this point I think modifying the html page manually to hard-code your link is sounding pretty good.
/* Get rid of the "(pdf, 438KB)" */
/* Final li on page, i.e. the "Play in Browser" link */