Okay, so I was browsing the internets again, looking for a nice multicart engine, until I came across...
THIS.Long story short, Damian Yerrick decided to make a follow-up to the so very much infamous Action 52 that would include a more interactive menu, as well as a bunch of great and not-so-really-great homebrew games (there are some big names on the list, like Shiru!).
Not to mention that the menu is available to the public and, moreover, is opensource... Which makes it a great base for a multicart, apart from those OneBus-based 200-in-1 cartridges. It requires some knowledge about how NES's memory works, though...