The national borders are almost always, if not always, relations. I checked the Austrian border and it is a relation describing it.

I downloaded it with:
http://www.informationfreeway.org/api/0.6/relation[name:en=Austria]

What you get, though, is the relation without it’s constituent parts. You then have to parse it and download each of the ways.

Anyhow, the id of the relation is 16239.