Interesting dilemma.
While not perfect, this got me pretty close: New Comp, New Layer -- I created a mask/path, then applied the Stroke effect. I set it the effect to Paint on Transparent, Hardness to 100%, Brush Size around 1.6 (this is the critical setting, which you'll want to adjust to taste). Then I created an Adjustment Layer with a Levels effect. Set the Level's filter Channel to "Alpha", then slide the Alpha Gamma to 0.00. This gave me a nice, nasty jagged line, with most parts (but not all) exactly one pixel wide (you'll have to precomp it if you want to put it on top of anything else). Adjust the Stroke Brush Size until you get the width you like best.
The only thing that comes to mind to get EXACTLY what you want it is to create the final pixelated line in photoshop, pixel for pixel. Then use your preferred method to reveal the line in AE (masking, painted mask, animated stroke, etc).
Keep in mind that often a single-pixel line isn't single pixel in the end, but aliased, with softened edges. Most people can't tell the difference; in fact, they will see an aliased line as single pixel more readily than a non-aliased one. Especially when it's a part of a motion graphics piece.
Ben Unguren
Motion Graphics & Editing
http://www.mostlydocumentary.com
