I wouldn't think that is a good idea for a couple reasons.
It has a very rough surface and most people who put bed liner on the outside of there truck use Line-X. Also it would most likely would not match your paint. Thirdly it would cost more then paint. Also weight might be an issue.
That's just some off the top of my head. The reason people get there rocker panels and bumpers Line-X'd is because it prevents scratches and rock chips, but you wouldn't have to worry about that at all with a tonneau or cap that was painted with straight paint.
Ford Ranger EDGE
Drivetrain: 8.8 LS w/ 4.10's
Exterior Mods: 06+ Emblems
Audio: 500 RMS Kenwood Amp, 10" Kenwood Shallow Sub