I don't know a general answer for your question, but I would be very excited to learn of one.  Or at least more examples!  Here is what I do know. 

In the forward direction - from the upper half plane $\mathbb{H}$ to the polygon $P$.  If $P$ is a regular $n$-gon, the SC map can given in terms of a hypergeometric function.  See Exercise 5.19 of the paper "Mappings to polygonal domains" in the book "Explorations in complex analysis".

In the backwards direction - from $P$ to $\mathbb{H}$.  If $P$ is a "rational" triangle (all angles are rational multiples of $\pi$) then it may be unfolded to give a quadratic differential $q_P$ on a Riemann surface $X_P$.  (The unfolding procedure comes from the theory of billiards.)  The inverse to the SC map is then, by the Schwarz reflection principle, a branched covering from $X_P$ to the Riemann sphere. In the presence of symmetries this covering map can have pretty expressions.  

I carefully understood the covering map in two cases beyond the trigonometric example.  When $P$ is a square or a hexagon we treat it as the unfolding of a rational triangle $Q$.  We deduce that $X_Q$ is a square or hexagonal torus.  Thus the covering map is the Weierstrass $\wp$-function for the square and hexagonal lattices, respectively.