Holy crap, thank you guys so much for offering your advice on this, and replying so quickly.
Open the file in a program like WindHex and use one of our tables to test the encoding. Go to Documents on the left and search for "Table" - you'll find 'em all there.
You'll probably have to use Cartographer and Atlus to dump and reinsert the script. Worst-case, if the scenario data really is encrypted or compressed, you'll need to code your own tools.
I tried every table on RHDN, and while some (namely, the Shift-JIS table) provide less messed-up results, it still looks to be encrypted. I'm trying out RLDev right now, but if that doesn't work, then this is certainly going to be an interesting project for me. (Especially since I haven't touched a drop of code in two years.) Still, if it turns out that there aren't any utilities that'll work with this particular file, then re-learning to code will at least be a fun learning experience.

Haeleth released his rldev toolset for the Reallive engine a few years back but he has since left the community so it's pretty outdated. Here's an updated fork in case Haeleth's version doesn't work. If *that* one still doesn't work you can try tracking down Richard_23 and ask him for rldev 1.51. I'm not sure if he's released it publicly but I know he made his own fork for the Little Busters! project which has additional encryption that Haeleth's version doesn't support.
As for the .cgm and .tcc files, I don't recall us ever having to edit those for the Kanon project so you probably don't need to touch them unless you have a specific reason for wanting to modify them. If you're looking for the images then they are usually in the G00 folder. rldev supports conversion to and from the g00 format so you're covered there, too.
I'm trying out the fork as we speak, and hopefully I'll be able to report back on that on the morning. Currently I'm feeling like a bit of an idiot for tying to look up what on Earth this error message is telling me to do.
Actually, I was just curious as to what the heck those files
were. I'd managed to figure out what most of the different file folders were for, but I didn't know if those two were something I'd need to worry about, so thanks for clearing that up. Also, seeing as you worked on Kanon, d'you mind if I ask whether that game had .gan files on the disc? I know they're some sort of scheduling/management program (which reads them as corrupt), but do VN companies usually leave them in there? The game I'm working on has a whole folder of 'em, and it seems a little odd to me, but then again, I don't know a lot about how companies do their work.
Is it possibly the one with the main character wearing eyeglasses?
If so, the text at the beginning of the game is in Seen0101.txt. Find any program that can display Shift-JIS encoding and you should see the text. Even internet browser like Firefox can display it if you have Shift-JIS supported. There are codes mixed in with text, so you will have to either find a program or write a custom program if you want to extract/insert text. (The dialog text isn't at the beginning of the file. You'll have to move down to see it.)
Yeah, I'm playing around with
Kichiku Megane right now. At the moment, all the Seen parts are compressed into a single file, though. After a little more research, I came across
this thread, which talks about the same problem with other RealLive games. Reading that thread definitely made me think, but I'm not sure if that's a good or a bad thing.
Spoiler: "1. The encryption of reallive games is done in two parts, masking some bytes the scenarios, and embed the key in the reallive executable and applying an out-of-box protection layer. So you can't find the key in the SEEN.TXT, and good luck trying to debug the copyright protected exe.

2. The key length is the same for all these games. Of course each game use a different key.
3. The main weakness of the protection scheme is that the encryption is too simple. As you may already know from the rldev source, it is just XOR.
Given another fact that reallive is a widely used engine by all companies (or workshops) under VisualArts, and not all released games are encrypted -- so the key is actually very easily recoverable by doing the following:
a. Get another unprotected game released around the same period
b. Extract a unencrypted utility scenario (these scenarios provides standard in-game services, and are game independent, and likely to be reused across multiple games)
c. Extract the encrypted utility scenario (usually having the same name/index) from your game
d. XOR them, now you get the key."
From what I can gather, both from those posts, and the fact that you've managed to view some of the individual, non-compressed Seen files, getting this to work is definitely doable. I just don't know how long it's going to take me to learn the skills to actually do it. Would you mind telling me how you accessed those indiviual files?
Anyway, sorry for the wall of text, everyone. I'm going to try some more things, fiddle around a little more, and see what happens. Right now, it's late, and I have a feeling I'm going to need to learn a
lot to tackle this project.