In a very rare update to the Diary section of his personal website, Shouji Sato gave us a bit of news to end what has been a barren stretch of no information. He announced Triage X Volume 6 will be out February 9. I guess this is the first official announcement, though volumes are usually announced earlier. He also announced that Volumes 5 and 6 of the Highschool of the Dead Full Color Edition will be released some time in the future. There is no date, but it appears he has begun working on them. Thus it seems Yen Press wasn't acting alone entirely, which is a good sign. This also means, however, that the ad seeking artists was possibly just looking for people to help color the new Full Color Editions. This wouldn't necessarily be proof he is not also working on new Highschool of the Dead. In fact, he could be hiring extra people to take over the coloring process entirely so that he can focus on drawing for two series again. We will have to wait and see.
I am almost certain he also talks about the HOTD manga itself in the post, but I am waiting on some translation help to see exactly what that is about. I can assure you there are no specific dates or time frames. I will update this post as soon as I have more information.
I don't really have much more information yet, but I did promise an update. The post by Shouji Sato does not mention HOTD returning. I saw "HOTD" and "thirty" in the post and got excited, but after further investigation, it appears he is simply pointing out how Triage X has passed thirty chapters, thereby surpassing HOTD in number of chapters. He notes his development as an author and artist while working on Triage X.
I hope to have more for you in my monthly Dragon Age update, which should come in a few days. See you then.