I lean towards the the simple/cheep solution.  Electronic are a neat idea
but seem to be expensive.  The metal detector sounds like a better solution.
Different metals give off different tones.  If you could tune a detector to
a unique metal not in the hive.  Set the scan area to be long and narrow.
You could search down through the hive, find the frame and pull it.  As for
metal tag.  Replace it with metal paint.  Their are metal paints available
that are basically glue with powered metal in them.  In addition, the paint
would give you a visual tag.