Ah, hentai. A guilty pleasure of mine, admittedly.
I've spent my fair share of time searching for CT hentai. Think what you will of me; I'm a completist and I enjoy porn.
In any case, the CT pickings are indeed slim. One phenomenon that I've noticed is that CT hentai is limited almost exclusively to Yuri pics-- girl on girl. CT is one of the only series to have such an overwhelming yuri fanbase. There's an obvious reason for this, in that the girls are the only *normal* people that the fanartists have to work with. I mean, for guys they have Magus, Crono, Robo and Frog-- and those last two aren't really what anyone wants to look at in a sexual context.
In the doujinshi I've collected, there's a touch of yaoi (specifically Magus/Crono and Glenn/Cyrus) and yaoi involving Flea/Magus (although anything involving Flea defies easy categorization) I have nothing hardcore, not even a kiss- just cuddling and overtones.
I also have a tiny bit of straight stuff. There's a japanese doujin fron '95 or so that I only own in digital form (hence it not being on my site) that features Crono/Lucca; I also have a sweet Lucca/Glenn and the usual gag setup pairings of Lucca/Magus and Crono/Marle.
Were I to quantify my findings of actual hardcore SEX images, I have found a total of 3.5 CT hentai full-length stories; Zone 8, of course, the cutepet Schala/Marle previously mentioned, the Lucca/Crono mentioned above, and a Lucca/Ayla I got about fifteen pages into before I stopped working on it. For pictures, I have about 30 saved on my HD; I've saved even the crappy ones, though, (completist) so to count the GOOD images cuts it down to 15.
Considering the size and scope of teh intarweb, that's pretty sad.