Apparently the staff did have a new doujinshi to give out (I wonder what it's called since Farewell Rakugaki Giga Drill was supposed to be the last staff doujinshi?). The cover is red with an illustration of Yoko.
From the Yoko thread: "Gurrepala had more Yoko than I thought it would..."
From the Nia thread: "The sense of Wonder was really great! Nia (Yukarin) isn't exactly singing, but the music is quite good
They said they spent 6 months making the music to match the work
Also very disappointing news...Ootsuka-san said that making the Nia BOX in present conditions would be difficult......orz"
(commence much weeping in Nia thread) D:
EDIT 2: More news from the Nia thread:
"When asked if the Nia BOX is going to come out,
I think how they answered is that Gurren Lagann has burned out with Lagann-hen.
They won't be producing anything related to Gurren Lagann for a while,
but if there's a lot of demand from fans, who knows...
Sorry it's just a vague memory"
And apparently I's CUBE does the vocals for The sense of Wonder?
(I's CUBE also did the song for Akemi Hayashi's AniKuri):