Doubt about Default Material

[micheus]

Is there no way to set the default material properties (things other than diffuse color, like specularity and opacity)

No. It's hard "coded". You can just play with diffuse as you already know.

and have them not get replaced with a new default material by Wings the next time you restart the program?

Well, default should also be a unique name in the material list and with the same settings as hard coded. So, every time you change the default material in your project, when you restart Wings3D it will be replaced by the "original" one and your previous changed default material will become something like default1, default2,...

It should not be a big deal, once you start creating new materials for your object.