I still think it would be a cool idea if it didn't involve soldering/routing all those wires through such a small space. It just wouldn't be worth the time and effort, especially for something as simple as a clock.
As for a clock cartridge that displayed the time on a TV, I have no idea how you'd do that. You mean something like a homebrew program that runs on the N64 itself, right? Doesn't sound feasible at all, to be honest. I don't know of any homebrew community to speak of for the N64 in the first place, and I think you'd need a flash cartridge (expensive) to get the program on there, in which case it's less of a clock cartridge than a clock program that happens to be on a flash cartridge. And if you're not playing a game in the first place, you probably have better things to do with the TV than check what time it is.
Maybe you could tap the video line of the N64 (can you output composite video with a microcontroller? I honestly have no idea) and install a switch to display the time when there isn't a game inserted, but again, that's throwing practicality out the window.
As for a clock cartridge that displayed the time on a TV, I have no idea how you'd do that. You mean something like a homebrew program that runs on the N64 itself, right? Doesn't sound feasible at all, to be honest. I don't know of any homebrew community to speak of for the N64 in the first place, and I think you'd need a flash cartridge (expensive) to get the program on there, in which case it's less of a clock cartridge than a clock program that happens to be on a flash cartridge. And if you're not playing a game in the first place, you probably have better things to do with the TV than check what time it is.
Maybe you could tap the video line of the N64 (can you output composite video with a microcontroller? I honestly have no idea) and install a switch to display the time when there isn't a game inserted, but again, that's throwing practicality out the window.