Question

Is it possible to (in code) access the metadata of a document stored in a library via a link to that document?

Answer 1

Get SPFile object using the SPWeb.GetFile method, then use the Item property of that object to get access to the ListItem accompanying the file.

using(SPSite site = new SPSite(link))
{
  using(SPWeb web = site.OpenWeb())
  {
   SPFile file = web.GetFile(link);
   SPListItem item = file.Item;

   object fieldValue = item["some field"];
  }
}