First semi-workable version in git.

Apart from miscalculated cutout corners it seems to be working.

envelope.asy

@@ -0,0 +1,95 @@
+from settings access outformat;
+outformat="pdf";
+unitsize(1mm);
+int pgw = 210;
+int pgh = 297;
+size(pgw, pgh);
+
+int sqsz = pgw;
+
+// C6: 114 x 162 mm
+int envh = 114;
+int envw = 162;
+string envfmt = "C6";
+// A6: 105 x 148 mm
+
+// debugging tool
+bool with_labels = true;
+
+
+// It is unpractical to try emulating second quadrant. We will work in the
+// first quadrant with the origin at the bottom-left corner of the square (not
+// the whole page). Therefore, the bottom edge of the page is at y=(210-297)
+int pgbot = 210-297;
+
+//Force A4 size
+//draw((0,pgbot) -- (0, sqsz) -- (sqsz, sqsz) -- (sqsz, pgbot) -- cycle, invisible);
+
+// gimme a square?
+if (with_labels) draw(box((0,0), (210,210)), p=red);
+
+pair envc = (sqsz/2, sqsz/2);
+// Write a paper type to the image?
+label(rotate(45)*scale(4)*Label(envfmt, envc, p=gray));
+
+pair corn_vec = unit((1,1)) * (envw/2) + unit((1,-1)) * (envh/2);
+pair flip(pair p) {
+	 return p - unit((1, -1)) * envh;
+}
+
+pair[] corners = {
+	envc + corn_vec,
+	envc + flip(corn_vec),
+	envc - corn_vec,
+	envc - flip(corn_vec),
+};
+
+// Draw the base rectangle
+draw(corners[0] -- corners[1] -- corners[2] -- corners[3] -- cycle, p=gray+dashed);
+if (with_labels) {
+	label("$E_0$", corners[0]);
+	label("$E_1$", corners[1]);
+	label("$E_2$", corners[2]);
+	label("$E_3$", corners[3]);
+}
+
+// Enlarge the sides:
+//llba: line at left bottom, A side
+pair llba = (1, 1);
+real llbb = dot(llba, corners[2]);
+pair llta = (1, -1);
+real lltb = dot(llta, corners[2]);
+pair lrba = (1, -1);
+real lrbb = dot(llba, corners[0]);
+pair lrta = (1, 1);
+real lrtb = dot(llta, corners[0]);
+
+pair lineinters(pair l1a, real l1b, pair l2a, real l2b) {
+	// For some reason, solve does not like having the matrices right in the
+	// parameters? (Or I made a stupid error in parentheses…)
+	real[][] a = {{l1a.x, l1a.y}, {l2a.x, l2a.y}};
+	real[] b = {l1b, l2b};
+	real[] solution = solve(a,b);
+	return (solution[0], solution[1]);
+}
+
+// Cutouts at all the sides:
+// Left: (cutout left first point)
+// Left edge: x = 0 = 1x + 0y
+pair cl1 = lineinters(llba, llbb, (1, 0), 0);
+pair cl2 = lineinters(llta, lltb, (1, 0), 0);
+// right
+pair cr1 = lineinters(lrba, lrbb, (1, 0), sqsz);
+pair cr2 = lineinters(lrta, lrtb, (1, 0), sqsz);
+// bottom
+pair cb1 = lineinters(llba, llbb, (0, 1), 0);
+pair cb2 = lineinters(lrba, lrbb, (0, 1), 0);
+// top
+pair ct1 = lineinters(llta, lltb, (0, 1), sqsz);
+pair ct2 = lineinters(lrta, lrtb, (0, 1), sqsz);
+
+// Draw the cutouts
+draw(cl1 -- corners[2] -- cl2);
+draw(cr1 -- corners[0] -- cr2);
+draw(cb1 -- corners[3] -- cb2);
+draw(ct1 -- corners[1] -- ct2);