I've now got contours rendering on http://openpistemap.org thanks to the very helpful Contours wiki page created by Gravitystorm (of cycle map fame) (osm.wiki/index.php/Contours).

I've taken a slightly different approach, which I think is a bit neater - instead of leaving the contours data as shapefiles I have imported it into PostGIS. I've updated the Contours wiki page with this alternative method.

The piste map is now rendering the whole planet data set rather than just one resort (but it won't update regularly yet), and it will render contours for most of the European ski resorts. I need to work out which bits of other continents I should be including contour data for (let me know if there's anywhere I've missed).