I end up having to do this to get Loader to set a base path correctly in configurator...

var loader = new Y.Loader({...});

// Set a base path if the user provides one (current.base is the base path the user enters in the text box), else default to yui.yahooapis.com/[version]/build/
if (current.base != "") {
loader.base = current.base;
} else {
loader.base = Y.Env.meta.base + Y.Env.meta.root;
}

loader.calculate();

a) The default base path doesn't seem to be accurate - hence the workaround in the "else" block (see #2528905).
b) Should loader pick up the base path from whatever is set for the YUI instance it's inside? Without the code in the "if" block above, it doesn't seem to.