I would like to inform you that we have just implemented the requested functionality, namely - if you enter a number, which is beyond the range of valid values, RadNumericTextBox will keep showing the value together with its InvalidStyle (the control will have no value). The behavior of the textbox will be controlled via a property and the default behavior will be the same as until present.
The feature will be included in the next service pack and nightly builds.
All the best,
the Telerik team