I think make_transform is suppose to construct a function suitable for transform_xy. Here's what I get:
(%i13) plot3d ([r, theta, sqrt (sin (theta))], [r, 0, 1], [theta, 0, 2*%pi], [transform_xy, make_transform ([r, theta, z], r*cos(theta), r*sin(theta), z)])$ Wrong argument for plot option "transform_xy". Expecting a symbol , true or false but found "#<COMPILED-FUNCTION $MAKE_TRANSFORM-1>".
Fixed by commit [1291fe], with the patch contributed by Tomio ((有坂 富雄). The correct command to show the use transform_xy would be something such as:
Related
Commit: [1291fe]