Basically - as discussed on Discord, I'm happy to implement this, if someone else is happy to make the character graphics. It should be fairly easy to implement this on the code side of things.
I'll look into doing it in a way that's fairly easy to add new characters to, so we don't have to modify an entire image or manually merge the images together, every time someone submits additional characters to it.
Further down the track, this could also be used to provide translations of NeoLemmix into other languages - but for now, let's just focus on (a) getting the support in and of itself, and (b) have it working in level title / author fields.
(Of course, I won't work on this right away - I'll probably come and take a look at what needs to be done with NeoLemmix, once I'm finished my Lemmings 3D levelpack.)