Is there a way to manually type the precise pixel position for a guide, instead of zooming in and dragging it to the correct position?
Sadly, no. I've wished for this many times myself.
Michael Szalapski wrote in a CreativeCow thread,
"At 100% zoom level in the comp viewer, guides move on full pixels, so you can place them on exact pixels right where you want them if you sit them on a null (or solid or whatever). Granted, it doesn't snap, but when I tried it just now, I got it right where I wanted it pretty quickly with a null as a reference. "
That's the best solution I've ever come across. Not what you asked for, I know.