Whilst it would make people more aware of the game engine, would it really be of much use to do this? It could be considered as extra clutter, for example, if I added a tag referring to an engine that only I knew about, would anyone really care, let alone know the answer to any questions on that tag?
Tags are about categorization of on-topic subject domains for searching and filtering. They are not about advertising or otherwise "making people aware" of products, and so should not be added for that purpose.
If an individual question is really about some engine (and isn't just a more general problem that happens to be done with or in that engine), it's certainly appropriate to add the tag to that one question. Unless the engine in question is a private, proprietary product, which case it's pointless.
It would not be appropriate to go back through the question history and retroactively apply that tag to anything that looks remotely relevant.