No. While the statement you quote is correct, doing this with iptables (instead of a proxy) would be very tricky.
Right now the iptables modules only monitor the data flowing through the router and depending on what it sees block it or redirect it, not alter it. This is even trickier because you want to add something -- if the size goes beyond the allowed packet size (above the MTU) there's a huge problem (though admitedly, I don't think this is likely with simple http requests, but it's still possible and an edge case that could cause problems).
Also, if you search using an encrypted connection (
https://google.com), the router won't pick anything up since the connection is encrypted.
Both of these problems can be circumvented using a proxy, but proxies eat up a lot of memory, which we don't have an small devices like the WRT54GL (only 16MB).