If the worst comes to the worst, you can drop the file onto TextEdit (It will open almost anything). However there will be a fair bit of gobbledegook code strewn about. But you'll see the text you want amongst it all none the less. But it sounds like you tried this already, more or less..
If there were any picts in the file, i'll doubt you'll see them without the correct version of iChat.
I doubt there is much you can do about it, until you have 10.4+ again..