Well,
there's no way around this that I know, the only thing you can do is turn this script into a vbscript snippet, and run it with wscript.shell's run method (if it is required that the output is written to file before any other processing).
Once your object.responsetext is filled with the html source, you can write it to a file using the FileSystemObject, where this +/-940 char limit doesn't exist.
So you'll end up with a text file with the requested data from the webpage.
I suppose you know how to do all this, if not let me know and I'll post an example.
Btw you know that mIRC supports sockets, right?
Greets
