I appreciate and use the guide lines in the Designer but there have been a few times when I can see having "custom" guide lines might be useful.
I used to do desktop publishing and the program we had, Page Maker, had horizontal and vertical guide lines that could be dragged in from the edges of the page to aid in alignment. Dragging from the edge of a form is probably not possible but perhaps there would be a way a user could overlay and position their own guides on a form. The way positioning the guides worked in Page Maker was you would place the mouse cursor on the guide line and the cursor would change to arrows pointing either left and right or up and down depending on the orientation of the guide and you would click-drag the line where you wanted it.
Just a thought.