I just posted a sample file in response to a request for help in the Autodesk Architectural Desktop 2005 Discussion Group, showing how a formula property can be used to return the value of one property - usually one of the automatic ones, but in this case a style-based manual property - unless an override value has been entered into a separate manual property, in which case the manual property is returned.
You would then use the formula property in your schedule [or tag], and the result displayed will be, in this case, the style-based manual property if the object-based override property is left at its default value, a null string. If the override property is not a null string, the override string is returned. The sample file includes a schedule table that displays all of the property data attached so you can see the effects of various combinations.
If you want to override a numeric value with another numeric value, I would suggest making the default value for the override property the number zero --> 0 <-- rather than a null string.