Over the weekend, Shouji Sato attended the AnimagiC 2012 anime convention in Bonn, Germany. The convention was July 27-29, and Shouji was there autographing on Saturday and Sunday. He also held 30-minute Q&A panels on both days. In an earlier post, I said it would be interesting to see if Shouji gives any news about the HOTD hiatus since the question would obviously come up. Well, after vigorous searching, I finally found the following information from the panel. It's a bit ambiguous, but it at least tells us more than anything in over a year.
As we expected, Shouji was asked about when the manga would return from hiatus. He answered that he is busy right now working on Triage X and doing research for future Highschool of the Dead chapters. This research involves studying weapons, vehicles, and other things so he can draw with the high level of technical detail for which he is known. He asks that fans be patient and give him time to bring us the same high quality we have come to expect.
This answer seems to put the long delay on him rather than Daisuke Sato. To me, it sounds like maybe Fujimi Shobo wanted Triage X to be monthly to boost its popularity, they chose to put HOTD on hiatus because it would be too much work for Shouji, and now he is doing the preliminary work for chapters Daisuke already has planned out so it will be less work when HOTD returns and Triage X doesn't then have to go on hiatus itself. That's just speculation, though it does sound more plausible than other theories I've had in the past. In any case, things are looking very hopeful now that we know work is being done on the series.
He was asked various other questions during the half-hour panel. When asked if any main characters will die (like in other zombie media), he replied that he doesn't know, which makes sense since he isn't the writer. He did say he hopes the main group will survive, though. He was asked how long Shido would be around in the future, to which he again responded that he doesn't know but hopes he will die soon. Someone asked him if he liked Takashi, and he does but says his favorite character is Kohta Hirano. This is obvious to me considering their shared fascination with guns.
In a question unrelated to HOTD, Shouji was asked when he started drawing. He said in grade school he became very ill and had to stay home for a long time. He couldn't do anything active, so he started to draw a lot and really liked it.
This information came from ~Abbadon82 at deviantART, an attendee of the convention. You can see the original post here.
Triage X Volume 4 will be ready very shortly. Look for it within the next day or two. See you soon!