Is there a way to configure this so that both the iMac and MacBook can back up to the Lacie as seamlessly as they did with the Time Capsule
Sorry, but no.
or am I stuck getting a plain old NAS device?
That is one option.
Another option.......if you are also looking for a new Wi-Fi router......would be the Synology router or one of the upper end Asus routers. These products support Time Machine backups to a hard drive connected to the USB port on the back of the router, so you can back up your Macs over the Wi-Fi network.....or.....connect the Macs directly to the router using an Ethernet connection.
The down and dirty option......Pick up an AirPort Extreme for $50-75 from Ebay and don't use it as a Wi-Fi router at all. Connect it to your existing router using an Ethernet connection and then connect the Lacie drive to the USB port on the back of the AirPort Extreme. The Macs can back up over your existing router's Wi-Fi network.
Backups will be slow with this option because the AirPort Extreme's USB port is USB 2.0.