While it's certainly no big deal to just change the ambient settings in the source ( Renderer.pas ) when you want the sole light source to have a different value then the current default ( C0.4 / A1.0 ), it might still be convenient being able to access the value dynamically from ZGE itself.
+Same goes for specularity actually .. and the Diffuse Color
*Although even better would be to have a Light Component obviously.
yeah a couple lights would roc! or even ANY setting for the current one,. or I should learn more shader coding,. eh. well I think simple stuff will get used as you add it, so I second the request for light switches,. er settings.
If you're not comfortable writing GLSL from scratch, take a look at RenderMonkey. Only tried it briefly myself, but it seems to have a workflow that doesn't require much coding ( + comes with a bunch of nice GL2.0 examples ).