A good tag line or opening line can be absolutely a cool way to organize a campaign, I would certainly be willing to try to run a campaign around such a line.
With this line, the "dragonmarked" could even have plenty of interpretations. It could simply refer to all the houses wanting to get him and thus, could even work with a campaign starting at 1st level. If the PCs happen to be marked too, so much the better, but otherwise they could simply be hired by representatives from all the houses. In fact, how badass must the man in black be if all of the Houses want him eliminated?
Granted, the PCs probably cannot go into the Mournland at first level but that doesn't mean the campaign cannot start at 1st. Perhaps the houses lost him after he faded into the Mournland and now they are trying to pick up the trail. Maybe there have been certain recent events that are characteristic of the Man in Black, etc.
Anyhoo, all kinds of options for something like this. The only issue you really have to be careful with is knowing how long you want the campaign to run and making sure that trail stays warm enough to keep the party's interest while making sure that the Man in Black continues to avoid capture, etc.