How does the clothing metaphor in the captain's account foreshadow Macbeth ripping the heart out of Scotland? "Till he unseamed him from the...

Thanks to the Captain's colorful reportage, Shakespeare's Macbeth begins and ends with a beheading: 

Till he unseamed him from the nave to th' chops,

And fixed his head upon our battlements.  (Act 1.2.22-23)

and a stage direction in Act 5.8:

Enter MACDUFF, with Macbeth's head.

The beheadings provide unity, then, enclosing the drama with identical violent acts.  The connection between the beheadings is so strong that although only the first one involves skewering the severed on to a spear and raising it above the battlements (in Shakespeare's text), Roman Polanski's film version features the same being done to Macbeth's head to powerful effect.

The tailor metaphor, "unseamed him," echoes other tailor imagery in the drama, most notably the tailor reference by the Porter:

...Knock, knock, knock.  Who's there?  Faith,

here's an English tailor come hither for stealing out of a French

hose [skimming on expensive fabric and substituting inexpenseive].  Come in tailor.  Here you may roast your goose.  (Act 2.3.10-12)

This imaginary character in the metaphorical hell of the castle is by his equivocation connected to the famous gunpowder plot, as would be the traitor Macdonwald--the traitor that Macbeth unseams--by Shakespeare's audience.  Macbeth, too, then, comes to be associated with the gunpowder plot.

Since Macbeth's sword would have obviously passed through Macdonwald's heart on its way to his chin, one could say that the metaphor might foreshadow Macbeth's tearing the heart out of Scotland.  That is probably not the primary focus of the metaphor, however.  Macbeth will trade places with the traitor Macdonwald, wearing clothes, and a crown, that do not quite fit.