If I understand you correctly then yes - you can have a splat as your default route. You would do something like this in your root shell:
router.map({
moduleId: "child/shell",
route: "*details"
});
And then in your child's view model:
var childRouter = rootRouter
.createChildRouter()
.makeRelative({ moduleId: "child" });
// Uses "child/defaultPage" as the view model, and "#/" as the route
childRouter.map({
moduleId: "defaultPage",
route: ""
});
Hope that helps.