I think there may be some confusion over stringtable values versus data fields in your question.  But I think your question raises an interesting point:  If you can change stringtable values dynamically, do you really want to?  Stringtable values / enviroment library settings are intended to be used to store value that might change ...

The environment library is a design-time construct.  At run time the values can be found in the string table.  Take a look at this link and see if this helps: http://k2underground.com/forums/thread/22180.aspx