Ok so answered my own question.. looks like the ContextID passed into DotNetDataSource is the current node ID. yay!

public class MyDocumentTypeProvider : IDotNetDataSource
{
    IEnumerable<KeyValuePair<string, string>> IDotNetDataSource.GetEditorDataItems(int contextId)
    {
        var cs = ApplicationContext.Current.Services.ContentService;
        var cts = ApplicationContext.Current.Services.ContentTypeService; 
        var myDocumentType = cts.GetContentType("MyDocumentType");

        if(myDocumentType != null)
        {
            var items = cs.GetContentOfContentType(myDocumentType.Id).Where(x => x.Id != contextId);
            if (items != null && items.Any()) {
                foreach (var item in items)
                {
                    yield return new KeyValuePair<string, string>(item.Id.ToString(), item.Name);
                }
            }
        }
        yield break;
    }
}