They had many participants in 2005 (19, winner Spike) and 2006 (20, winner Shredder). I am not sure, but I think those were open tournaments where engines/programmers could apply for. Hiarcs did not participate.
Two weeks ago, a computer chess fan posted an info about the 2007 tournament on the CSS message board. It seems that the Chess Tigers organizers do not care much to bring their infos to the typical computer chess sites, and/or fora.
Later, I found these messages on another message board:
http://f51.parsimony.net/forum204700/messages/130.htm
http://f51.parsimony.net/forum204700/messages/131.htm
The former Tiger's board member mentions financial constraints and that they invited only four programs, this year.
About Hiarcs being the Chess960 rating list leader at CCRL, I am not entirely sure that the Chess Tigers organizers even know that list. Basically, I would assume that they would have considered to of course invite Hiarcs. I guess they simply don't know that list.
("Chess960" doesn't appear on that CCRL 40/4 FRC page which may cause a problem if somebody searches for Chess960 but not for FRC.)
Maybe Hiarcs should challenge the Mainz winner for a longer match
like in the FIDE/Rybka affair.