Yes, you’ll be able to upload custom emojis/reactions. The library supports it and we will build UI tools for it in fiture updates.
as for reaction trigger - yes, current setup is like that by design. As we use it more we will look for further improvements.
I would be curious on how this would be possible currently. is there a folder on the server that the images would just be uploaded to?
By default, the all media files are stored in separate folder in Stroage directory on yout server as other medai files from UNA. It is not a problem to add any number of other storage folders to save custom Emoji or any other images.