I think this would need to be integrated into web browsers as a standard or as an extension. You have a chunk of money you've prepaid to a service, and that service lets you pay (makes you pay?) $0.05 for viewing an article, and remembers that you did it, and the paywall gives you the option to instantly pay with PayService.
Of course then PayService will have surge pricing turned on. And that will be fun. But still - viable?