I did some searching in the mailing list, and I guess people edit $:/core/templates/static.content; to include opengraph tags. I can PR that in myself.
We also like to make Fission apps PWAs by default, but that could be a little tricky with all the iFrames? I’ll leave that for the future.
I got this to work by making a new tiddler, content-type text/plain, and adding the tag $:/tags/RawMarkup to it. From: TiddlyWiki — a non-linear personal web notebook
Maybe you know this already, or I’m misunderstanding what you need? But this page is what showed up when I googled something similar. It helped me have other apps & sites see opengraph metadata for my own site: Thanael's Site — A Single Homepage App