No you can't (name servers don't work that way) unfortunately and likewise I can't recommend any specific free hosts as I personally have no need for such a setup, it was merely an idea.
What you guys might do, just before the move, is setup a C record for your domain (instead of the default A record) and point that to your free host addy (assuming it's a subdomain) and then on whatever free host you're using you could setup an .htaccess file with a rewrite rule in place to redirect any requests to the default "Sorry we're down" page.
You'd have to actually find a free host that allows you to setup an .htaccess file which also has mod_rewrite enabled, but I imagine there has to be some out there. There are a lot "if's" in this suggestion as they're all based off of assumptions that your free host provider will give you a subdomain and allow you to use .htaccess and mod_rewrite. Hence it'd take you ages to even find a free host that offers those exact features.
Another possibility which would be much easier, but is not completely free ($25/yr), would be to again setup your primary domain with a C record and point it to a no-ip.org registered domain name (You have to be using their "Plus" package to use the "Offline" features). Their offline feature will redirect any requests to either another host or an "offline" page of your choosing. If I needed redundancy, I'd probably go that route myself, as it'd be a lot more reliable than a free host.
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