My guess is its impossible to do unless you can make yourself invulnerable (I haven't tried overdosing on Fire Resist potions and armor), which only Vampire Lords can do with the Mist Perk. Note that even while invulnerable the Magma still causes fire to appear around your screen and really clouds your vision. Jumping while flying across will alleviate this and allow you to get your bearings. Also note that as a normal Vyre the heat from floating over the magma will kill you in seconds. *EDIT* You can also use the Become Ethereal Shout to cross the Magma safely as well. This is much more difficult as you cannot jump high enough to end the obscuring fire effect.
[img]http://i1224.photobucket.com/albums/ee378/_AsWeWept/0701120131.jpg[/img]
The exit from the magma lake hidden area can be seen by turning right while facing the Forge.
[img]http://i1224.photobucket.com/albums/ee378/_AsWeWept/0701120126.jpg[/img]
The location of the first chest as seen by standing to the left of the Forge.
[img]http://i1224.photobucket.com/albums/ee378/_AsWeWept/0701120128.jpg[/img]
The first chest.
[img]http://i1224.photobucket.com/albums/ee378/_AsWeWept/Untitled-1.jpg[/img]
Although it is difficult to see; This gate opens into a hidden passage that ends with you exiting through the gate in picture 1.
[img]http://i1224.photobucket.com/albums/ee378/_AsWeWept/0701120129.jpg[/img]
The gate seen while standing on a small rock that can be seen from standing to the right of the Forge. All the rocks can be stood on safely, but they're very small. You can also see one of the downward inclines into the Magma here that are on both sides of the Forge.
[img]http://i1224.photobucket.com/albums/ee378/_AsWeWept/0701120130.jpg[/img]
The room right after the gate seen in pictures 4 and 5.
[img]http://i1224.photobucket.com/albums/ee378/_AsWeWept/0701120130a.jpg[/img]
The second chest. Continuing down the passage will lead you to the exit in picture 1.
Goodluck!