Hey Ke,
This depends on what mod you're playing on (as Dune said).
On the most servers, where you're able to change your eye emote you have to use following chat command:
"/emote EMOTE_TYPE TIME_IN_SEC"
How about DDrace? Does it work there?
Yes, DDrace servers added this feature (at least at V0.6.x. I think on 0.7.x they have not)
So if you want to change your eye emote for a long time on these servers you just have to write:
"/emote happy 999999"
That are about 700days of happy eyes (should be enough )
(Of course after re connection you have normal eyes again, but you just have to use this chat command once again and you're having happy eyes again )
I hope I could help you!
~Anker