Here's a quick workaround, that doesn't require the use of another program:
Since we make sure the line is drawn from edge, to edge, when we go to rotate it, ArtRage puts its axis at the dead center of the page. Because of this, we can be assured that the 1% pixel line will also be dead center on the page, when turned upright.
1 - Let's assume you want to split the page in half vertically. Using the Ink Pen tool (set to 1% size), draw a line horizontally, from the left edge to the right edge. It's important that the line is perfectly straight, and fully reaches the borders of the page on both sides, in order to work. To ensure this, start and end the line off the edges of the canvas, and use the Ctrl+Shift+Left Click drag method to make it a perfectly straight line along the horizontal. It doesn't really matter where the line is made, but the closer to center, the better.
2 - Once you've created the line, use the Transform Tool to select and rotate the line, so that it's perfectly Vertical (Hold the Shift key while rotating, to lock the line to precise degree angles).