I see two sides to this.  First, most dealer test plots are not research plots, they are are show plots.  Keep this in mind:  The purpose of a seed dealer plot is to sell product.  The sponsor (be it dealer or company) really does not want to give the competition any free advertising. 

Now, if you want to prove the superiority of your product, or do a variety to variety head-to-head comparison, that is great.  Set up replicated research type plots and do it.  But keep the public out of them.  When I wander through knocking over stalks, cutting them off and splitting them, and peeling back ears ... I am affecting the outcome of the plot.  Set up a show plot for customers to look at and replicated trials to prove things.

One other problem with a plot is the sheer number of varieties!  I just got my new Beck's purchasing guide in the mail.  They have over 70 production varieties, not counting any new releases for next year that a plot should include.  70+.  How do I decide which is a fair comparison to my Pioneer 3186?

I have decided if a seed dealer is to really do justice to a seed plot he is goig to have to have enough seed volume to justify taking 40 acres and making it into plots.  One block RR, one block RW, one block CB, one block Clearfield, etc and etc.  Maybe even one of POC (Plain Old Corn).  There are just too many choices these days.

Put it in short 4 row strips.  Make it a place his customers can look at the products.  Then have a research area with replicated strips including competition he wants to compare against.  But control access to these, making them look from the end only.

The photo below is way too much for a dealer ... it's a company plot looking at a lot of things besides just varieties.  But the basic principles are still true. A farmer plot is for sales, not research.