THis could not possibly be more OOC.

I recognise "marriage" (結婚), but what's the rest of the text in the lower right?

It's not only not out of character, it's in fact official art from the fourth novel.

Inviting Kyon to join the literature club, if I remember right.

glasnost said:
I recognise "marriage" (結婚), but what's the rest of the text in the lower right?

It's Chinese. Someone edited it into the original picture (which should be posted, if possible).

If no one has a better scan, http://www.baka-tsuki.net/project/index.php?title=Image:Sh_v04_illust05.jpg could be used.

glasnost said:
I recognise "marriage" (結婚), but what's the rest of the text in the lower right?

That's basically it. Nagato (or rather, whoever put in the Chinese text) is just suggesting that the two of them get hitched.