Unless you hacked the router, it wouldn't send any packets at all. It would never bring the IP link up because address negotiation had failed. Even if you did get it to accept packets, no packets bound for that address would ever get back to you.
Perhaps you are confusing the router with the modem.
There
is no address negotation when static setups are used. Your router will send packets out on an IP that it is not assigned to if you set it up to do so, address negotiation is bypassed. You can hook your computer directly to your modem without a router and assume any IP, the router is just a hardware specific computer.
The modem on the other hand or the ISP gateway further down the line will filter these out.
You are of course correct that even if the ISP failed to filter the packets you sent out that your response would never make it back.
When ISPs were young and they did not filter so well you used to be able to spoof an IP and send a PING packet out to a thousand computers and they would all PING back to the IP you are spoofing. It was a way to hide your DOS attack, this of course does not work on the modern internet.
