Yes, plug your Gigabit LAN port into your NAS, and then go to System Preferences > Network > Action menu [The "gear" icon button at the bottom of the interface service list] > Set Service Order...
...and then drag AirPort above Ethernet in the list. Then click Okay and then Apply.
Ordering your interface services this way tells your machine that AirPort should be your default interface instead of Ethernet. Ethernet will still work for getting to the local subnet (i.e. your NAS), but AirPort will be used for getting "out to the Internet" (this is known as your "default route").
Oh, you'll also probably need to figure out IP addressing for this two-device subnet of you and your NAS. Since this is now an isolate network since your HomePlug repeaters are down, your NAS and your Mac probably can't get to a DHCP server on this network, so you'll need to give them static IPs on the same subnet (but not on the same subnet as the rest of your home network), or you'll need to fire up a DHCP server on this network. If your NAS can serve DHCP, you can have it do it. Or you could turn on Internet Sharing in Mac OS X "From: AirPort, To: Ethernet", to have your Mac act as the DHCP server so your NAS can get an IP lease from your Mac.