Hi Kyle and Bob,
If the resolution does not appear available then there is a chance the refresh rate or other graphics setting may not be supported by the monitor. I recommend contacting the manufacturer of the computer in order to check if you need to set additional graphics settings besides resolution to make it work.
This link has more information about custom modes http://www.intel.com/support/graphics/sb/CS-029478.htm
Can you clarify something for me real quick? In the GW2 game settings menu, does the 1280x720p resolution not show up for you in game? If so, are you saying that you are experiencing noticeable performance improvement by launching the game from a desktop setting of 1280x720, playing the game in 1280x720p, and then (mid-play) changing your desktop resolution back to its native? Is this performance increase noticeable in terms of FPS? (You can check FPS in game by hitting F11 and looking in the bottom right corner of the settings menu).