The A6 engine defaults to A5 mode on slower video cards. This mode displays objects with some of the new material effects and shaders as transparent (Enviroment Reflection or Normal Mapping for example). I believe this is because these shaders are incorporating a .tga with alpha channel, and when the shader doesn't work correctly, the model just picks up the transparency from the alpha.

What's the best way to create a fallback such that the model simply reverts to opaque in A5 mode?

The best way is to check in which mode the engine runs.

I don't think that material effects are even supported by a5.

Exactly. So if a player has an older video card, the engine will automatically default to A5 Mode, and all the models that looked really cool under A6 Mode, now look mosty transparent. So don't you need to put something into the material that says, "Hey, the current player's video card can't render this effect then ignore the alpha channel in the models bitmap."

But how do you do that?