No joy on getting the full 1366x768 but after quite some fiddling I've at 
least got 1360x768 working on the Motorola Atrix Lapdock, representing a 
significant improvement over the defaults.

BeagleBone Black version

I have a BeagleBone revB (with the 2GB on-board storage) and used the Angstrom 
Distribution (BeagleBone Black - 2GB eMMC) 2013-09-04 
<https://s3.amazonaws.com/angstrom/demo/beaglebone/BBB-eMMC-flasher-2013.09.04.img.xz>
 
from http://beagleboard.org/latest-images/

/etc/angstrom-version
Angstrom v2012.12 (Core edition)
Built from branch: angstrom-staging-yocto1.3
Revision: 2ac8ed60f1c4152577f334b223b9203f57ed1722
Target system: arm-angstrom-linux-gnueabi

/media/BEAGLEBONE/ID.txt
 
Cloud9 GNOME Image 2013.09.04



Linux Terminal: uEnv.txt

To improve the resolution of the ttys outside of X11 (available via 
CTRL-ALT-F1, ... FN), add the video parameter to /media/BEAGLEBONE/uEnv.txt 
as shown:

/media/BEAGLEBONE/uEnv.txt
optargs=quiet drm.debug=7 capemgr.disable_partno=BB-BONELT-HDMI video=HDMI-A
-1:1360x768M@60

You can check current kernel params via cat /proc/cmdline

X:  /etc/X11/xorg.conf

The EDID info for the lapdock's monitor doesn't seem to have timings / 
resolutions that work.  After fiddling, for awhile I found two modeline 
improvements over the 720x576@50 mode that required editing xorg.conf. 
 Note that I defined a "Monitor" section for the "MotoAttach" identifier, 
then pointed the existing "Screen" definition to this new monitor.  You 
could also drop the Modelines into the default monitor config and leave the 
"Screen" alone.

 Here's my entire /etc/X11/xorg.conf:
Section "Module"                                           
        Load    "extmod"                                
        Load    "dbe"                           
        Load    "glx"                                           
        Load    "freetype"                             
        Load    "type1"                        
        Load    "record"                    
        Load    "dri"                       
EndSection                                


Section "Monitor"                                                           
             
        Identifier      "Builtin Default Monitor"
EndSection                                           


Section "Monitor"
    Identifier "MotoAttach"
    VendorName "MOT"
    ModelName "MotoAttach"
    HorizSync 30-85
    VertRefresh 50-75


    Modeline "1360x768@60" 84.50  1360 1392 1712 1744  768 783 791 807 +hsync 
+vsync
    Modeline "1280x720@50" 74.25  1280 1720 1760 1980  720 725 730 750 +hsync 
+vsync
EndSection




Section "Device"                                      
        Identifier      "Builtin Default fbdev Device 0"
        Driver  "modesetting"
EndSection                                                    


Section "Screen"                                            
        Identifier      "Builtin Default fbdev Screen 0"     
        Device  "Builtin Default fbdev Device 0"            
        #Monitor "Builtin Default Monitor"               
        Monitor "MotoAttach"               
    DefaultDepth    16
EndSection                                                  


Section "ServerLayout"                                     
        Identifier      "Builtin Default Layout"                           
          
        Screen  "Builtin Default fbdev Screen 0"        
EndSection


These pages were helpful to varying degrees in finding some Modelines that 
work better:

   - http://elinux.org/Beagleboard:BeagleBoneBlack_HDMI#Default_Resolutions
   - http://www.arachnoid.com/modelines/
   - http://www.epanorama.net/faq/vga2rgb/calc.html
   
EDID info wrong?
Here's the reported EDID info, which does not work when put into xorg.conf, 
gathered from this command:

parse-edid /sys/class/drm/card0/card0-HDMI-A-1/edid
parse-edid: parse-edid version 2.0.0
parse-edid: EDID checksum passed.


 # EDID version 1 revision 3
Section "Monitor"
   # Block type: 2:0 3:ff
   # Block type: 2:0 3:fd
   # Block type: 2:0 3:fc
   Identifier "MotoAttach"
   VendorName "MOT"
   ModelName "MotoAttach"
   # Block type: 2:0 3:ff
   # Block type: 2:0 3:fd
   HorizSync 30-85
   VertRefresh 50-75
   # Max dot clock (video bandwidth) 150 MHz
   # Block type: 2:0 3:fc
   # DPMS capabilities: Active off:no  Suspend:no  Standby:no


   Mode "1366x768" # vfreq 60.000Hz, hfreq 48.000kHz
     DotClock 72.000000
     HTimings 1366 1380 1436 1500
     VTimings 768 769 772 800
     Flags "+HSync" "+VSync"
   EndMode
   # Block type: 2:0 3:ff
   # Block type: 2:0 3:fd
   # Block type: 2:0 3:fc
EndSection


Change X resolution from command line
These commands are convenient to execute over SSH or serial connection 
while flailing at xorg.conf modelines.

*Failsafe mode*
export XAUTHORITY=`ls /var/run/gdm/auth-for-root-*/database`
export DISPLAY=:0.0
xrandr --output HDMI-0 --mode 720x576 --rate 27


*Restart X*
(faster than rebooting)
systemctl restart display-manager


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