Friend Leaderboards - Request "after rank"

I managed to solve the problem. I will share my solution here in case it helps anyone else.

When dealing with the Friends Leaderboard, I download the entire leaderboard and then selectively show whatever I want. To download the entire leaderboard:

Leaderboards.GetEntries( szLeaderboardName, 100, LeaderboardFilterType.Friends, LeaderboardStartAt.Top ).OnComplete( OnFriendLeaderboardDataReceived );

100 is the maximum entries you can request at a time so, we must handle the possibility that the user has more than 100 friends (lucky):

void OnFriendLeaderboardDataReceived( Message<LeaderboardEntryList> msg )
{
	if( !msg.IsError )
	{
		LeaderboardEntryList entries = msg.Data;

		// Store the entries here (code removed for brevity)

		// We need to check if there is more data...
		if( m_NumItemsDownloaded < (int)entries.TotalCount )
		{
			// Request more data
			Leaderboards.GetNextEntries(entries).OnComplete( OnFriendLeaderboardDataReceived );
		}
		else
		{
			// All data downloaded
		}
	}
	else
	{
		Error error = msg.GetError();
	}
}

The nice thing is - you can test the code by limiting the initial call to GetEntries to just 1.

Hope that helps someone.