I don't see the point in this.
If a thread is important, it wouldn't 'fall back'. If it fell back, it's obvious that it's not important enough to stay up top.
And if it is really that important to make a reply, then just make a reply? :\
Could be just me assuming things here, but I think the only reason why you want this is to bump your own AMA thread or something. Which obviously isn't important.