We support exactly this[1].

http://wym.io/

Incidentally I almost had a heart attack when I saw the Lambda announcement. :P

[1]: We also provide the ability to respond to other kinds of events as does Lambda.

havent looked too much into it but I dont see why it couldnt. use s3 website, event notification to notify a user accessed a file (web page) and generates a new custom file in your bucket. generated file has some sort of preshared known info between the two in the file name (eg client ip address or cookie). accessed web page either sends back a page reload to the new generated page or just an async page that will render the custom content.

edit: oh also delete the generated file once accessed using lambda by either delete on get notification or just a lambda scheduled event.