This will force the enter key inside hangouts in place the smiley key (on applicable keyboards of course). No it will not be just a visual change, it will be the ACTUAL enter key so that you can type multi-line messages.
This should also work on any future versions of Hangouts because of the way the variables are obtained. Of course, post if it does not work on a future version of Hangouts.
Download attached apk or download from the Xposed module repository.