On Gimp 2.8, the "Comics-o-matic" GIMP plugin by ilguido can do this. However, it is not designed for single guides, so you get a few extras that are easy to remove.
Download the plugin (.scm file) and put it in your
.gimp-2.8/scripts directory, then restart GIMP. (On Windows 7,
c:\users\<yourname>\.gimp-2.8\scripts.) Open your image and go to the new "Comics-o-matic" menu. Select
Add Guides (framing). You will see the following dialog box:
Set "Columns" and "Rows" to 1 and then specify your guide positions in the top, bottom, left, and right margin boxes. You can use mm or 32nds of an inch (pick in the Units box). If you only want one guide, put its position in the Top margin (for horizontal) or Left margin (for vertical), and use 0 for the other dimensions. Hit OK and you will get four guides for your margins. Drag the ones you don't want off the image using the Move tool. Dragging off is especially easy if you selected margin 0, because you only have to move them a pixel outside the image to make them disappear.