Add interfaces for custom TabAPI, just need to add the hooks now, and of course a pretty example. As always, not tested yet.

api/src/main/java/net/md_5/bungee/api/TabAPI.java

@@ -0,0 +1,58 @@
+package net.md_5.bungee.api;
+
+/**
+ * Represents a custom tab list, which may have slots manipulated.
+ */
+public interface TabAPI
+{
+
+    /**
+     * Blank out this tab list and update immediately.
+     */
+    void clear();
+
+    /**
+     * Gets the columns in this list.
+     *
+     * @return the width of this list
+     */
+    int getColumns();
+
+    /**
+     * Gets the rows in this list.
+     *
+     * @return the height of this list
+     */
+    int getRows();
+
+    /**
+     * Get the total size of this list.
+     *
+     * @return {@link #getRows()} * {@link #getColumns()}
+     */
+    int getSize();
+
+    /**
+     * Set the text in the specified slot and update immediately.
+     *
+     * @param row the row to set
+     * @param column the column to set
+     * @param text the text to set
+     */
+    void setSlot(int row, int column, String text);
+
+    /**
+     * Set the text in the specified slot.
+     *
+     * @param row the row to set
+     * @param column the column to set
+     * @param text the text to set
+     * @param update whether or not to invoke {@link #update()} upon completion
+     */
+    void setSlot(int row, int column, String text, boolean update);
+
+    /**
+     * Flush all queued changes to the user.
+     */
+    void update();
+}

proxy/src/main/java/net/md_5/bungee/tablist/Custom.java

@@ -0,0 +1,130 @@
+package net.md_5.bungee.tablist;
+
+import net.md_5.bungee.api.TabAPI;
+import com.google.common.base.Preconditions;
+import java.util.Collection;
+import java.util.HashSet;
+import net.md_5.bungee.api.ChatColor;
+import net.md_5.bungee.api.TabListHandler;
+import net.md_5.bungee.api.connection.ProxiedPlayer;
+import net.md_5.bungee.protocol.packet.PacketC9PlayerListItem;
+
+public class Custom extends TabListHandler implements TabAPI
+{
+
+    private static final int ROWS = 20;
+    private static final int COLUMNS = 3;
+    /*========================================================================*/
+    private final Collection<String> sentStuff = new HashSet<>();
+    /*========================================================================*/
+    private String[][] sent = new String[ ROWS ][ COLUMNS ];
+    private String[][] pending = new String[ ROWS ][ COLUMNS ];
+
+    public Custom(ProxiedPlayer player)
+    {
+        super( player );
+    }
+
+    @Override
+    public synchronized void setSlot(int row, int column, String text)
+    {
+        setSlot( row, column, text, true );
+    }
+
+    @Override
+    public synchronized void setSlot(int row, int column, String text, boolean update)
+    {
+        Preconditions.checkArgument( row >= 0 && row < ROWS, "row out of range" );
+        Preconditions.checkArgument( column >= 0 && column < COLUMNS, "column out of range" );
+        Preconditions.checkNotNull( text, "text" );
+        Preconditions.checkArgument( text.length() <= 16, "text must be <= 16 chars" );
+        Preconditions.checkArgument( !sentStuff.contains( text ), "list already contains %s", text );
+        Preconditions.checkArgument( !ChatColor.stripColor( text ).equals( text ), "Text cannot consist entirely of colour codes" );
+
+        pending[ROWS][COLUMNS] = text;
+        if ( update )
+        {
+            update();
+        }
+    }
+
+    @Override
+    public synchronized void update()
+    {
+        clear();
+
+        for ( int i = 0; i < ROWS; i++ )
+        {
+            for ( int j = 0; j < COLUMNS; j++ )
+            {
+                String text;
+                if ( pending[i][j] != null )
+                {
+                    text = pending[i][j];
+                    sentStuff.add( text );
+                } else
+                {
+                    text = new StringBuilder().append( ChatColor.COLOR_CHAR ).append( base( i ) ).append( ChatColor.COLOR_CHAR ).append( base( j ) ).toString();
+                }
+                getPlayer().unsafe().sendPacket( new PacketC9PlayerListItem( text, true, (short) 0 ) );
+                sent[i][j] = text;
+                pending[i][j] = null;
+            }
+        }
+    }
+
+    @Override
+    public synchronized void clear()
+    {
+        for ( int i = 0; i < ROWS; i++ )
+        {
+            for ( int j = 0; j < COLUMNS; j++ )
+            {
+                getPlayer().unsafe().sendPacket( new PacketC9PlayerListItem( sent[i][j], false, (short) 9999 ) );
+            }
+        }
+        sent = new String[ ROWS ][ COLUMNS ];
+        sentStuff.clear();
+    }
+
+    @Override
+    public synchronized int getRows()
+    {
+        return ROWS;
+    }
+
+    @Override
+    public synchronized int getColumns()
+    {
+        return COLUMNS;
+    }
+
+    @Override
+    public synchronized int getSize()
+    {
+        return ROWS * COLUMNS;
+    }
+
+    @Override
+    public boolean onListUpdate(String name, boolean online, int ping)
+    {
+        return false;
+    }
+
+    private static char[] base(int n)
+    {
+        String hex = Integer.toHexString( n );
+        char[] alloc = new char[ hex.length() * 2 ];
+        for ( int i = 0; i < alloc.length; i++ )
+        {
+            if ( i % 2 == 0 )
+            {
+                alloc[i] = ChatColor.COLOR_CHAR;
+            } else
+            {
+                alloc[i] = hex.charAt( i / 2 );
+            }
+        }
+        return alloc;
+    }
+}