Two things. Interface availability has been added to 10.2 per the release notes...I'm awaiting my change window to allow me to install to utilize this feature myself. See here for more details.
Secondly I as well have the need to identify the difference in WAN Interfaces. What I did was to create a custom property simply called WANInterface. This is a True/False property and I tag all WANInterfaces in this method. While I believe it would be nice for Solarwinds to identify some of this automatically I feel that may be difficult with the number of Ethernet handoff circuits I'm seeing from carriers. While these are "WAN" connections they are now on Ethernet ports not serial so my method allows me to report on these as well.
Using that WAN Interface property allows me to create custom reports and filter other resources to show information only on my interfaces that have been tagged.