Could the script.txt be kept (or compressed) into under 2000 characters?
In that case, the Spring installer could set Spring up as an URL handler. So your lobby would do the launch by offering the user a link of the form
<a href="spring:myreallylongscript.txtcontents">Start Game</a>
Windows provides the means for an installer to provide a new URL handler, although on Linux it's handled by the desktop environment and I've only ever seen it done on Gnome-based desktops.
See this article:
http://kb.mozillazine.org/Register_protocol
Now, I don't know if you can do a Javascript redirect to a non-HTTP url, so you might require manual intervention there...
but if Spring registered an URL handler, you'd be in awesome shape for a weblobby.
Edit: Dansan's idea of a script.txt with a dedicated mim-type is better.
Registering a mime-type file association to launch an app is much easier than registering an URL handler.