I'm not much help here since I've been thinking the same thing and do not have an answer.  Does anyone make a dry fertilizer cart that would work well to pull behind a 16 or 24 row corn planter?  I'm thinking something that would hold at least 5 tons and fill quickly.

The old style boxes work fine if you are not trying to apply a high rate per acre and have a good filling system.  It would seem some of the things used in strip tillage systems could be tweaked a little to work with a planter.