All nodes in a corridor template drive breaklines in the corridor model. The breaklines all act as "Sharp and Texture Boundaries". You can apply Surface Textures to the corridor surface in the same way you apply them to a normal surface. You create the Materials in the MSI Manager as you suggested. e.g. Create a Material Category called Pavement Materials and then create a material called Asphalt. The Asphalt Material can be assigned a color or a Texture File. The Texture Files can be found online using Google - there are thousands of them out there.
Use the Create Surface Texture command - that will ask you for the Layer - we place a point on that layer that carries the properties of the texturing and acts as a seed point to flood all the triangles with the selected material color or texture file. You name the Texture e.g. Asphalt Pavement and then select the Material (the list is generated from the MSI Manager). You will find that the color or material texture will flood the selected area until it hits a breakline in the model or a Template Drop in the model (across the model).
You will therefore need to texture the left pave and the right pave as separate "texture locations" because the centerline will act as a Texture Boundary.
All Linestrings have the Surface Sharpness Property and if you want the linestring to act as a Texture Boundary then you need to set that property to "Sharp and Texture Boundary".
The Material when selected if left blank / none, will null the triangles in the model between breaklines set to Sharp and Texture Boundary if you texture the surface with "no material". Name the Material you place to something like Null.
I place my Texture Points on the layer called SITE - SI Markers.
Hope this helps - will try to record a video for you next week.
Note surface areas with different surface textures will report separately if you run a Surface Information Report and you can use ths to compute seeding area quantities etc. if necessary.