Let $ABC$ be a triangle with $AB=7$, $BC=9$, and $CA=4$.  Let $D$ be the point such that $AB\parallel CD$ and $CA\parallel BD$.  Let $R$ be a point within triangle $BCD$.  Lines $\ell$ and $m$ going through $R$ are parallel to $CA$ and $AB$ respectively.  Line $\ell$ meets $AB$ and $BC$ at $P$ and $P^\prime$ respectively, and $m$ meets $CA$ and $BC$ at $Q$ and $Q^\prime$ respectively.  If $S$ denotes the largest possible sum of the areas of triangle $BPP^\prime$, $RP^\prime Q^\prime$, and $CQQ^\prime$, determine the value of $S^2$.

Let's think step by step and output the final answer within \boxed{}.
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