It's been mentioned before, just not very recently, don't worry.
It's technically possible, at least for Steam, especially now that you can grab the information via Steam's OpenID implementation. But that would involve a lot of coding, and matt already has quite a bit on his plate... Not to mention Steam is very, very bad about actually showing what you own (For example, I have the SSFIV AE costume pack with everything, but it doesn't show up in my profile, and even the store thinks I don't own it!) And they tend to change game IDs around at times.
As far as I know, there's no publicly-accessible way of viewing a person's game collection on Origin, other than through the client's built-in friends list - I didn't know that until just now. Uplay does have a public game list but their site is still clunky as hell.
Other sites like Impulse (Gamestop), GreenManGaming, Amazon, etc. are just game stores and they have no way of showing people what certain accounts already own. So there's no point in having a way to add their games automatically.
Possible? Yes. Practical... not very.