I have a couple H2GO models that are 6.7oz and another that is 7.1oz. The published weight I found for the 40oz wide-mouth Klean Kanteen was 9.7oz— not an earth-shattering difference, but ~30% would be considered respectable with any other UL gear.
One source that I forgot that is a re-branded H2GO bottle: http://www.planetzerogravity.com/cmg/?q=beverage-bottle/1-liter

I suspect there is a limit to the wall thickness because of the forming process and wouldn't expect to see stainless bottles that are much lighter. You can actually flex the wall of the H2GO bottle a bit with your hand. At 6.7oz, you are at par with a plastic Nalgene. Existing titanium pots in the 700-900ml range are 4oz-6oz, so I don't know how much farther you can expect to go from 6.7oz for a steel bottle. Note that they are 40oz capacity, which is 1182ml and that weight included the plastic lid and retaining ring.
The lids are removable like a Nalgene and the bottle can be used on a stove or in a fire, so it it is possible to recoup some of the weight by using the bottle as a boiling pot. It would be great to see one that is a little wider and with thinner gauge metal for use as a pot. A titanium version would be better yet. Something about the width of an MSR Titan with a removable screw-top plastic cap would work for me. I would add a small bail or loop to help haul it off a stove or fire and to aid carrying.
I originally bought mine to use as an all-in-one survival kit. The metal container allows cooking and boiling for water purification. The bottle is a waterproof container and the wide mouth allows storing all kinds of items inside. When it comes time to use the bottle for hauling water, the contents could go in your pockets. A little paracord would allow carrying the bottle bandoleer-style.
Re: cannibalizing a Thermos bottle. The wall thickness on the thermos is at least as thick as the H2GO bottles. Are you thinking of using the inner wall and the threaded stopper? It may work with the Steripen, but it will be close. I'll bet that it will end up about the same diameter as the narrower-necked Klean Kanteen and at least as heavy if not more so.