An example of what he was looking for, he said in a later Tweet, was the cover made for the packaging of Monster World IV, a rather lovely action RPG that was released for Sega's Mega Drive back in 1994.
Sadly, that prompt didn't help a whole lot--at least at first. In fact, the only piece of cover art I could think of was the one that was produced for the Japanese version of Advance Wars: Dual Strike, which was known as Famicom Wars DS in that territory.
Later, though, a few others came to mind, such as Teketeke! Asmik-kun World (aka Boomer's Adventures in Asmik World) for the GameBoy and HAL Laboratory's Eggerland (part of the series that later became known as Adventures of Lolo) for the Famicom Disk System.
The Eggerland box art (below) is among the most delicious things you've ever laid eyes on, right? Don't be shy--admit it.
The only other example I've been able to think of, by the way, is Atlus' Totsugeki! Valetions (see it here), which also is a Japanese GameBoy title. (Its name was changed to Spud's Adventure when it was brought to North America in 1991.)
I don't suppose any of you can think of examples of game cover art that showcases toys or figures instead of drawing or renders? If you can, please let me know about them in the comments section of this post.