"It would be ok as long as the cans weren't stored upside-down."

I'd shared this preprint unread when it popped up on the arxiv but apparently it's good enough for Nature News, which has a good writeup. I particularly enjoyed the caveats.

The widget-free way to foamy stout

Stouts bubble less readily than lagers or other carbonated drinks when poured because they contain dissolved nitrogen as well as the carbon dioxide that drives the fizz. Adding nitrogen makes the beer less acidic, and gives a longer-lasting head with relatively small bubbles that are behind stouts' smooth, creamy 'mouth feel'.
But the addition also demands the use of the widget [a hollow sphere with a hole in it], which takes in gas and beer as it floats in the canned stout and, when the can is opened and the pressure drops, jets it out again through the hole, helping to create the foam.

The new study suggests that the same result could be achieved by coating part of the can's interior with cellulose fibres. [to increase the rate of bubbling, which is intrinsically low for stout: bubbles nucleate faster on rough surfaces]
[...]
However, [some guy] adds, the bubbles might fill with liquid while the can was in storage. Lee says the coating would be placed in the gap at the top of the can: "It would be ok as long as the cans weren't stored upside-down."

Andrew Alexander, a chemical physicist at the University of Edinburgh, UK, also believes that can coatings would be impractical compared with widgets. "Widgets are genius — they're cheap, work really well, are totally non-toxic and don't mess with the beer," he says. "Would a fibrous coating be cheaper than what is essentially a ping-pong ball with a hole in it? I don't think so."
But Lee says that using widgets slows down the process of canning stout. "Oxygen stuck in the widget can affect the beer's flavour, so you have to pump nitrogen in several times to remove it," he says. "The cellulose coating is an alternative worth investigating."
However, it is likely to be some time before fibre-lined stout cans appear on supermarket shelves. "We've spoken to brewers," says Lee, "but we're not sure if they're interested yet."