Personally, I would LOVE to have the option of showing character portraits in the Text Creator.
The reason why it hasn't been done yet is because those portraits, under the hood, are deeply tied to the mission system that the engine uses for playsets and missions like the ones in the Toy Box Hub. Decoupling those portraits from the current mission system, and add them to the Toy Box, is NOT a trivial process, and doing it right obviously takes quite a bit of time.
It could have been done for 3.0, but that would have meant taking out replayable missions from playsets and the ability to get a maximum of 3 stars. This feature was made because we got a lot of requests asking for adding more replayability in playsets. And well, that feature got a bigger priority over the portraits.
Obviously, this doesn't mean that it will never happen, but rather I wanted to explain why it hasn't happened yet. We'll definitely make a push for those in the future, because I completely agree that they would make the Toy Box feel more like a playset!
The opinions posted here are my own, and not my employer's.