Added note: The procedure below assumes your initial hull surface is intended to be developable.
First, make sure the overall surface is a true developable surface which works with Unroll and UnrollUV. It is generally extremely difficult to create true developable surfaces by lofting through multiple stations. Note that except in special cases the stations on a boat with developable surfaces will have curved, not straight lines.
DevSrf (free plug-in from McNeel) is a good tool for creating developable surfaces from two curves such as a sheer and chine curves. The curves will probably need to be extended and the resulting surface trimmed to cover the full length of both surfaces.
Create a curve where one side of the rectangle is desired on the developable surface using CurveOnSurface and only the beginning and end points, no intermediate points. Such a curve will unroll straight. Use ExtendCrvOnSrf to extend the curve to the edges of the developable surface. If desired place a point on the curve at the desired location of one corner of the rectangle.
Use UnrollUV to unroll the overall surface and curve (including corner point if desired).
The side of the rectangle should be a straight line on the unrolled surface.
Create the other side and ends of the rectangle where desired on the flat surface.
Use FlowAlongSrf to put the second side and ends of the rectangles on the developable surface.