Why dont using Inkscape type of guides? (its free, install it and try guides) Just for inspiration.
Its just like the guides in photoshop vertical or horizontal but it has a point somewhere on the line that you can drag to re-position
the line, also this point can be moved on the line (the point can be moved horizontally on an horizontal guide, vertically on a vertical one)

Why not making clines that work that way with a button in the ui...

1. Click on the cline button.
2. Click on a first point, then on a second point, an infinite line is drawn between the two.
optional 2.1 (to snap the two points, you could also use the current clines that are already in MoI (the help lines) to help you positioning the two points))
3. You can move these points at any time just like other points, the line is always straight.
4. But you can also move the two points at the same time, or the whole line to move it parrallels.

Basically it would be like click'n'drag like the actual help lines in MoI but it would create lines that stay, also it would be cool
to be able to change the color in the MoI options. And these lines would be auto-locked, to move it instead of click just click once on it to delock and show the 2 points, then click on points to move them or on the line to move it or select the two points and move, if you click or select any other things the line would just lock itself automatically.

Also using a shortcut that show/hide clines would be very cool, like the photoshop default one ctrl-h cmd-h.