Dewaxed shellac for knots
Sure.  The shellac makes a barrier that the tannins can't get through.  If you shellac over primer, it's still effective.  I have verified this through experience.  

As a side note, an oil based primer (like Kilz Original) that blocks tannins by itself will, for most applications, be sufficient to block knots from bleading through.  It's also easy to sand out brush marks with the oil-based stuff.