I have just uploaded a new version of the Rotation Constrained script for Blender 2.8 as of the build revision available today, and it's available on the scripts page or from here. As the Blender 2.8 Python API is still ongoing changes this script may well break in the near future. If so post in the comments below or at the github page https://github.com/rgsouthall/rotation_constrained/issues.
This version also features improved handling of rotations in Global, Local and Normal space, which are selected from the the top of the 3D view. Please note that in Normal space 'Z' is always normal to the face, but the Rotation Constrained 'X' and 'Y' directions are calculated on the fly and may not match up with the 'X' and 'Y' directions shown in the 3D view. You may therefore need to select 'X' in the Rotation Constrained operator interface when you mean 'Y' and vice versa.
Lastly the keyboard shortcut for the add-on is now 'alt-shift-R' to avoid conflict with another shortcut.
I'll post a video when I can demonstrating the basic operation.