Yes. I would love to make custom signs like Minecraft. It would be super useful for labeling things. Basically give us a blank space to write a small note in.
Personally, I like to run Vanilla servers and games. There could be a mod for just about anything, but then you risk people not having the mod, server incompatibility, incompatibility with other mods, and the creator of the mod dropping it then a patch that breaks it. In a game where numbers (for planet location) are so important, a writing system should be a built in feature. It also allows you to leave behind texts for other explorers who visit the same planet.