Still, I think it would be nice to have official support for this feature.
I agree. It would be nice to use vanilla JavaScript to access card metadata like “ease%” and “last review date”, but I don’t think these are available as an API for JS.
These seem to be the only “vanilla” metadata you can access: (source)
The note's tags: {{Tags}}
The type of note: {{Type}}
The card's deck: {{Deck}}
The card's subdeck: {{Subdeck}}
The type of card ("Forward", etc): {{Card}}
The content of the front template
(only valid in back template): {{FrontSide}}
Actually, I found this comment that might be useful.