Redfern: Yes, technically. But there is one possible problem (i think, not sure). Probably (i'm not sure) he will need to claim that page first, thus ceasing your access to that page. Or, maybe not. In that case you can try it with him :)
Senteria: I've actually done it before. I just don't want to get in trouble with the humble bundle police. :P
Oh, so its dont necessary to claim page to "tie it to steam account"? Great :)
As for policy, i think literally nobody will notice if you dont yell about it to Humble support :)))
If they allow to gift steam keys without losing corresponding downloads i dont see why they should care in this, similar case.
Actually, i think origin of lack of such option is only one - Store and Widget is more "direct" sales, where developer can dictate his will, while bundles is usually big mess where nobody from game developers controls much.
So, give it a shot. In other case your Steam key will just rot for eternity in your library.
