Why not?

src/cmd/map/libmap/mercator.c

@@ -0,0 +1,36 @@
+#include <u.h>
+#include <libc.h>
+#include "map.h"
+
+static int
+Xmercator(struct place *place, double *x, double *y)
+{
+	if(fabs(place->nlat.l) > 80.*RAD)
+		return(-1);
+	*x = -place->wlon.l;
+	*y = 0.5*log((1+place->nlat.s)/(1-place->nlat.s));
+	return(1);
+}
+
+proj
+mercator(void)
+{
+	return(Xmercator);
+}
+
+static double ecc = ECC;
+
+static int
+Xspmercator(struct place *place, double *x, double *y)
+{
+	if(Xmercator(place,x,y) < 0)
+		return(-1);
+	*y += 0.5*ecc*log((1-ecc*place->nlat.s)/(1+ecc*place->nlat.s));
+	return(1);
+}
+
+proj
+sp_mercator(void)
+{
+	return(Xspmercator);
+}