That doesn't make any sense. The keyed power source will trigger the relay, yes..but the pump still needs a power source and ground. You have not circumnavigated a potential problem, only added an extra circuit to it.
There is a bank on the fuse block that is protected by the fusable link, fed by the key, and intended to carry amp loads similar to what an electric fuel pump will draw.
Piggy back from that and all is well.[/
ok man, relays are stupid. That make you happy?