Hello @TJ Ballard
I followed your advice and created a simple test project which has one Blueprint and one structure. In the blueprint I set the Forward Axis vector of a spline mesh using a ESplineMeshAxis variable to demonstrate that the variable type is available in blueprints. In the structure I created one single variable and tried setting it to the ESplineMeshAxis type. Unfortunately I was unable it to the ESplineMeshAxis type just as I was unable to in my original project. The project file can be found here.
I guess I should also note that I am using the 4.8.3 release of the Unreal Engine.
Please let me know if you need anymore information and thank you for your help,
Farshooter