If you want you can explicitly set the filenames (and extensions) of log files for specific windows using the /log command. It's not very practical if you want to share logs for all your windows though.
I find it a bit of a strange option to want to be honest. The only reason you need it is because of an extremely specific and unlikely situation here since neither web servers nor web browsers care about file extensions, they only care about MIME types, so it's only the extension-based blocking at the school that's posing a problem. And for that there's a very simple answer that doesn't require an obscure feature for mIRC: Use mod_rewrite with a .htaccess file in your mIRC logs directory to make Apache map .txt extensions in requested URLs to .log extensions on the server.
I know you were asking for a feature rather than asking for another way to solve the problem but that seems like a far more appropriate solution than adding a feature that I suspect very few people would have a use for. Then again, it's not up to me, so we'll just have to wait and see if it's in the next version of mIRC.
