Will need a more complete example. We parse using statements (you see them in the Defs tool window) and we factor the using statements in the current file in when doing Context Tagging.
The only real limitation is that if you write side-effecting include files with using statements in them, all bets are off, until we start doing full preprocessing, we won't be able to detect that a using statement hidden in an include file has anything to do with the current file.